asyncio: addr_sync: maybe fix "no current event loop in thread"

see https://github.com/spesmilo/electrum/issues/5376
crash report for electrum 4.2.1:
```
Traceback (most recent call last):
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/python-installs/Electrum/kivy/base.py", line 347, in mainloop
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/python-installs/Electrum/kivy/base.py", line 391, in idle
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/python-installs/Electrum/kivy/base.py", line 342, in dispatch_input
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/python-installs/Electrum/kivy/base.py", line 308, in post_dispatch_input
  File "kivy/_event.pyx", line 724, in kivy._event.EventDispatcher.dispatch
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/python-installs/Electrum/kivy/uix/behaviors/button.py", line 179, in on_touch_up
  File "kivy/_event.pyx", line 720, in kivy._event.EventDispatcher.dispatch
  File "kivy/_event.pyx", line 1263, in kivy._event.EventObservers.dispatch
  File "kivy/_event.pyx", line 1147, in kivy._event.EventObservers._dispatch
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/python-installs/Electrum/kivy/lang/builder.py", line 57, in custom_callback
  File "<string>", line 42, in <module>
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/gui/kivy/uix/dialogs/wallets.py", line 70, in cb
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/gui/kivy/main_window.py", line 710, in load_wallet_by_name
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/gui/kivy/main_window.py", line 728, in on_open_wallet
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/gui/kivy/main_window.py", line 691, in on_wizard_success
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/wallet.py", line 3267, in __new__
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/wallet.py", line 3111, in __init__
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/wallet.py", line 2904, in __init__
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/wallet.py", line 288, in __init__
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/address_synchronizer.py", line 97, in __init__
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/wallet.py", line 402, in load_and_cleanup
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/address_synchronizer.py", line 106, in load_and_cleanup
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/util.py", line 439, in <lambda>
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/util.py", line 435, in do_profile
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/address_synchronizer.py", line 432, in load_local_history
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/address_synchronizer.py", line 533, in _add_tx_to_local_history
  File "/home/user/wspace/electrum/.buildozer/android/app/electrum/address_synchronizer.py", line 548, in _mark_address_history_changed
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/other_builds/python3/arm64-v8a__ndk_target_21/python3/Lib/asyncio/locks.py", line 260, in __init__
  File "/home/user/wspace/electrum/.buildozer/android/platform/build-arm64-v8a/build/other_builds/python3/arm64-v8a__ndk_target_21/python3/Lib/asyncio/events.py", line 639, in get_event_loop
RuntimeError: There is no current event loop in thread 'GUI'.
```

electrum/address_synchronizer.py

@@ -23,7 +23,6 @@
 
 import asyncio
 import threading
-import asyncio
 import itertools
 from collections import defaultdict
 from typing import TYPE_CHECKING, Dict, Optional, Set, Tuple, NamedTuple, Sequence, List
@@ -71,6 +70,7 @@ class AddressSynchronizer(Logger):
     """
 
     network: Optional['Network']
+    asyncio_loop: Optional['asyncio.AbstractEventLoop'] = None
     synchronizer: Optional['Synchronizer']
     verifier: Optional['SPV']
 
@@ -185,6 +185,7 @@ class AddressSynchronizer(Logger):
         if self.network is not None:
             self.synchronizer = Synchronizer(self)
             self.verifier = SPV(self.network, self)
+            self.asyncio_loop = network.asyncio_loop
             util.register_callback(self.on_blockchain_updated, ['blockchain_updated'])
 
     def on_blockchain_updated(self, event, *args):
@@ -560,10 +561,14 @@ class AddressSynchronizer(Logger):
                     self._history_local[addr] = cur_hist
 
     def _mark_address_history_changed(self, addr: str) -> None:
-        # history for this address changed, wake up coroutines:
-        self._address_history_changed_events[addr].set()
-        # clear event immediately so that coroutines can wait() for the next change:
-        self._address_history_changed_events[addr].clear()
+        def set_and_clear():
+            event = self._address_history_changed_events[addr]
+            # history for this address changed, wake up coroutines:
+            event.set()
+            # clear event immediately so that coroutines can wait() for the next change:
+            event.clear()
+        if self.asyncio_loop:
+            self.asyncio_loop.call_soon_threadsafe(set_and_clear)
 
     async def wait_for_address_history_to_change(self, addr: str) -> None:
         """Wait until the server tells us about a new transaction related to addr.