bundle libsecp256k1 in android apk

this works as there is already a p4a recipe upstream: 74bf788a29/pythonforandroid/recipes/libsecp256k1/__init__.py

gui/kivy/tools/buildozer.spec

@@ -31,7 +31,7 @@ version.filename = %(source.dir)s/contrib/versions.py
 #version = 1.9.8
 
 # (list) Application requirements
-requirements = python3crystax==3.6, android, openssl, plyer, kivy==master
+requirements = python3crystax==3.6, android, openssl, plyer, kivy==master, libsecp256k1
 
 # (str) Presplash of the application
 #presplash.filename = %(source.dir)s/gui/kivy/theming/splash.png

lib/ecc_fast.py

@@ -37,6 +37,8 @@ def load_library():
         library_path = 'libsecp256k1.dylib'
     elif sys.platform in ('windows', 'win32'):
         library_path = 'libsecp256k1.dll'
+    elif 'ANDROID_DATA' in os.environ:
+        library_path = 'libsecp256k1.so'
     else:
         library_path = 'libsecp256k1.so.0'