build: android: bump gradle version to work with Java 21

When building on debian 12, we were using Java 17.
On debian 13, there is Java 21.

Gradle was giving build errors with new Java:
```
Could not compile build file '/home/user/wspace/electrum/.buildozer_qml/android/platform/build-arm64-v8a/dists/Electrum/build.gradle'.
> startup failed:
General error during conversion: Unsupported class file major version 65

java.lang.IllegalArgumentException: Unsupported class file major version 65
```

see https://docs.gradle.org/current/userguide/compatibility.html

for our p4a fork, this cherry-picks:
- kivy/python-for-android@17bf532
- kivy/python-for-android#3172
    kivy/python-for-android@a8f2ca1

Author: SomberNight

contrib/android/Dockerfile

@@ -219,10 +219,11 @@ RUN cd /opt \
     && /opt/venv/bin/python3 -m pip install --no-build-isolation --no-dependencies -e .
 
 # install python-for-android
-ENV P4A_CHECKOUT_COMMIT="a01269f7799587ad74ee40e0b642d917b8db7d4e"
+ENV P4A_CHECKOUT_COMMIT="846a1094874aeb64b623fa746222a41851245271"
 # ^ from branch electrum_20251211 (note: careful with force-pushing! see #8162)
+# FIXME xxxxx tmp git repo
 RUN cd /opt \
-    && git clone https://github.com/spesmilo/python-for-android \
+    && git clone https://github.com/sombernight/python-for-android \
     && cd python-for-android \
     && git checkout "${P4A_CHECKOUT_COMMIT}^{commit}" \
     && /opt/venv/bin/python3 -m pip install --no-build-isolation --no-dependencies -e .