android build: use spesmilo/ fork of p4a and buildozer

- fdroid maintainers asked that releases use fixed forks of p4a and buildozer,
  so now we use the newly created forks in the spesmilo org. I plan to keep using
  my existing contributor-specific repos for development, but whatever we push
  to electrum master, should use the new spesmilo/ forks.
  see https://gitlab.com/fdroid/fdroiddata/-/merge_requests/15858#note_2136345746
  - Also, I added branch protection rules for branches named "electrum_*", so if
    we name the branches that are actually used in releases as such, we won't
    accidentally force-push them. (ref https://github.com/spesmilo/electrum/issues/8162)
    I think we can just create a new branch whenever we would want to force-push
    the existing one.
- also factored out some parameters so that it is easier to programmatically
  access them from the fdroid build script.
  see https://gitlab.com/fdroid/fdroiddata/-/merge_requests/15858#note_2136094409

contrib/android/Dockerfile

@@ -81,10 +81,11 @@ RUN apt -y update -qq \
 RUN yes | ${ANDROID_SDK_MANAGER} --licenses > /dev/null
 
 
+ENV ANDROID_SDK_BUILD_TOOLS_MAJOR_V="31"
 ENV ANDROID_SDK_BUILD_TOOLS_VERSION="31.0.0"
 
 # download platforms, API, build tools
-RUN ${ANDROID_SDK_MANAGER} "platforms;android-31" > /dev/null && \
+RUN ${ANDROID_SDK_MANAGER} "platforms;android-${ANDROID_SDK_BUILD_TOOLS_MAJOR_V}" > /dev/null && \
     ${ANDROID_SDK_MANAGER} "build-tools;${ANDROID_SDK_BUILD_TOOLS_VERSION}" > /dev/null && \
     ${ANDROID_SDK_MANAGER} "extras;android;m2repository" > /dev/null && \
     chmod +x "${ANDROID_SDK_HOME}/cmdline-tools/bin/avdmanager"
@@ -180,25 +181,21 @@ RUN /opt/venv/bin/python3 -m pip install --no-build-isolation --no-dependencies
     -r /opt/deterministic-build/requirements-build-android.txt
 
 # install buildozer
+ENV BUILDOZER_CHECKOUT_COMMIT="4403ecf445f10b5fbf7c74f4621bf2b922ad35b5"
+# ^ from branch electrum_20240930 (note: careful with force-pushing! see #8162)
 RUN cd /opt \
-    && git clone https://github.com/kivy/buildozer \
+    && git clone https://github.com/spesmilo/buildozer \
     && cd buildozer \
-    && git remote add sombernight https://github.com/SomberNight/buildozer \
-    && git remote add accumulator https://github.com/accumulator/buildozer \
-    && git fetch --all \
-    # commit: from branch electrum-qt6 (note: careful with force-pushing! see #8162) \
-    && git checkout "4403ecf445f10b5fbf7c74f4621bf2b922ad35b5^{commit}" \
+    && git checkout "${BUILDOZER_CHECKOUT_COMMIT}^{commit}" \
     && /opt/venv/bin/python3 -m pip install --no-build-isolation --no-dependencies -e .
 
 # install python-for-android
+ENV P4A_CHECKOUT_COMMIT="7197c1c28409fbeebd8494093349a2bfd770526a"
+# ^ from branch electrum_20240930 (note: careful with force-pushing! see #8162)
 RUN cd /opt \
-    && git clone https://github.com/kivy/python-for-android \
+    && git clone https://github.com/spesmilo/python-for-android \
     && cd python-for-android \
-    && 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/electrum_20240930 (note: careful with force-pushing! see #8162) \
-    && git checkout "7197c1c28409fbeebd8494093349a2bfd770526a^{commit}" \
+    && git checkout "${P4A_CHECKOUT_COMMIT}^{commit}" \
     && /opt/venv/bin/python3 -m pip install --no-build-isolation --no-dependencies -e .
 
 # build env vars