android build: update p4a/buildozer to latest upstream

Rebased our branches on latest p4a develop HEAD and buildozer master HEAD.

Re `android.allow_backup = False` in buildozer.spec, now that this setting
has been exposed to buildozer.spec (see https://github.com/kivy/buildozer/pull/1206),
we could finally drop a commit from our branch that did the same:
7a7d415d90

Re commenting out `android.arch` in buildozer.spec, we are setting this
via the ENV VAR in make_apk.

contrib/android/Dockerfile

@@ -160,8 +160,8 @@ RUN cd /opt \
     && cd buildozer \
     && git remote add sombernight https://github.com/SomberNight/buildozer \
     && git fetch --all \
-    # commit: from branch sombernight/202104_android_adaptiveicon
-    && git checkout "0ce292fabec299c78c8ffeaf42072ab879f29d8a^{commit}" \
+    # commit: from branch sombernight/electrum_20210421
+    && git checkout "c17ac3618334c9936253e8f5b88dce43dc4da75b^{commit}" \
     && python3 -m pip install --user -e .
 
 # install python-for-android
@@ -170,8 +170,8 @@ RUN cd /opt \
     && cd python-for-android \
     && git remote add sombernight https://github.com/SomberNight/python-for-android \
     && git fetch --all \
-    # commit: from branch sombernight/electrum_20200703
-    && git checkout "a4059599211a87af895d9ee2223f052a406357ca^{commit}" \
+    # commit: from branch sombernight/electrum_20210421
+    && git checkout "5356bc7838b03c8c174c91fe01539c91d1b40b9f^{commit}" \
     && python3 -m pip install --user -e .
 
 # build env vars