android: fix back button not working (main surface loses focus)

fixes #6276

contrib/android/Dockerfile

@@ -151,6 +151,7 @@ RUN cd /opt \
     && cd buildozer \
     && git remote add sombernight https://github.com/SomberNight/buildozer \
     && git fetch --all \
+    # commit: kivy/buildozer "1.2.0" tag
     && git checkout "94cfcb8d591c11d6ad0e11f129b08c1e27a161c5^{commit}" \
     && python3 -m pip install --user -e .
 
@@ -160,7 +161,8 @@ RUN cd /opt \
     && cd python-for-android \
     && git remote add sombernight https://github.com/SomberNight/python-for-android \
     && git fetch --all \
-    && git checkout "257cfacbdd523af0b5b6bb5b2ba64ab7a5c82d58^{commit}" \
+    # commit: from branch sombernight/electrum_20200703
+    && git checkout "0dd2ce87a8f380d20505ca5dc1e2d2357b4a08fc^{commit}" \
     && python3 -m pip install --user -e .
 
 # build env vars

electrum/gui/kivy/main_window.py

@@ -849,6 +849,17 @@ class ElectrumWindow(App):
             return
         self.use_change = self.wallet.use_change
         self.electrum_config.save_last_wallet(wallet)
+        self.request_focus_for_main_view()
+
+    def request_focus_for_main_view(self):
+        if platform != 'android':
+            return
+        # The main view of the activity might be not have focus
+        # in which case e.g. the OS "back" button would not work.
+        # see #6276 (specifically "method 2" and "method 3")
+        from jnius import autoclass
+        PythonActivity = autoclass('org.kivy.android.PythonActivity')
+        PythonActivity.requestFocusForMainView()
 
     def update_status(self, *dt):
         if not self.wallet: