ci: persist Gradle/Android caches across CI runs, give sccache headroom

GRADLE_USER_HOME and ANDROID_USER_HOME were scoped to the ephemeral git
checkout instead of the persistent cache dir, so every CI run re-downloaded
the Gradle distribution and the entire Maven/AGP/Kotlin/Compose dependency
cache from scratch. Move them alongside CARGO_TARGET_DIR/SCCACHE_DIR under
the persistent cache directory. Also bump SCCACHE_CACHE_SIZE past its
default 10GiB, which was sitting exactly at the ceiling and causing LRU
eviction on the Rust side.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MYfqVnUhRV9fPEP9gZpMqL

scripts/build-apk-ci.sh

@@ -14,12 +14,13 @@ APK_PATH="$REPO_ROOT/$APK_NAME"
 CACHE_DIR="${STATE_DIRECTORY:-/var/lib/gitea-runner}/cache"
 mkdir -p "$CACHE_DIR"
 
-export GRADLE_USER_HOME="$REPO_ROOT/.gradle-home"
-export ANDROID_USER_HOME="$REPO_ROOT/.android-home"
+export GRADLE_USER_HOME="$CACHE_DIR/gradle-home"
+export ANDROID_USER_HOME="$CACHE_DIR/android-home"
 export CARGO_BUILD_JOBS=2
 export CARGO_TARGET_DIR="$CACHE_DIR/cargo-target"
 export RUSTC_WRAPPER="$SCCACHE_BIN"
 export SCCACHE_DIR="$CACHE_DIR/sccache"
+export SCCACHE_CACHE_SIZE="20G"
 
 cat > "$REPO_ROOT/android/local.properties" <<PROPS
 sdk.dir=$ANDROID_SDK_DIR