fix: remove hardcoded macOS-only org.gradle.java.home from gradle.properties

[pbertsch]

Summary

android/gradle.properties hardcoded org.gradle.java.home=/Library/Java/JavaVirtualMachines/temurin-21.jdk/Contents/Home — a path that only exists on one macOS laptop, never on flutter-probe's Android E2E CI runner (ubuntu-latest). This broke every Android E2E Gradle build with:

* What went wrong:
Value '/Library/Java/JavaVirtualMachines/temurin-21.jdk/Contents/Home' given for
org.gradle.java.home Gradle property is invalid (Java home supplied is invalid)

Found immediately after fixing the nightowl_sdk local-path pub-resolution failure over in AlphaWaveSystems/flutter-probe#216 — that fix got the build past flutter pub get (confirmed passing on iOS), which is what exposed this second, independent issue on the Android side.

Removing the hardcoded path lets Gradle fall back to the JAVA_HOME environment variable (or PATH), which is how both CI's actions/setup-java step and any normal local dev setup already provide a JDK. No functional change for anyone who wasn't relying on that one specific hardcoded path.

Test plan

• flutter build apk --debug --dart-define=PROBE_AGENT=true still succeeds locally with the property removed, resolving a JDK via the normal Flutter/Gradle toolchain lookup
• Will confirm the Android E2E job actually goes green in flutter-probe's CI once this merges (triggering workflow_dispatch on the e2e.yml workflow there)