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

fixes spesmilo#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 "4f7bdf1bd2811b8a06a12f9d56bd5b18594caf60^{commit}" \
     && python3 -m pip install --user -e .
 
 # build env vars