android build: p4a to build pyqt5 using "hostpython" instead of sys py

The current system python in the docker baseimage is py3.9, while the
targetpython and hostpython built by p4a are py3.8, and this was causing
linker issues in the pyqt5 build.
It is also cleaner IMHO to have p4a handle what is needed for a recipe
instead of assuming it is available in the system. (and I think this is
how other existing recipes work)

fixes https://github.com/spesmilo/electrum/issues/8016

contrib/android/Dockerfile

@@ -176,10 +176,11 @@ RUN cd /opt \
     && git remote add sombernight https://github.com/SomberNight/python-for-android \
     && git remote add accumulator https://github.com/accumulator/python-for-android \
     && git fetch --all \
-    # commit: from branch sombernight/qt5-wip
-    && git checkout "c6e39ae1fb4eb8d547eb70b26b89beda7e6ff4b6^{commit}" \
+    # commit: from branch sombernight/electrum_20210421d
+    && git checkout "4535a333f9928e894caedefc4f746f9c6a0dc50f^{commit}" \
     && python3 -m pip install --no-build-isolation --no-dependencies --user -e .
 
 # build env vars
 ENV USE_SDK_WRAPPER=1
 ENV GRADLE_OPTS="-Xmx1536M -Dorg.gradle.jvmargs='-Xmx1536M'"
+#ENV P4A_FULL_DEBUG=1