Build scripts incompatible with Java 9

The packaged Android.mk is incompatible with Pie builds since the prebuilt JDK packaged with the ASOP is now version 9, which the included gradle scripts do not support. This leads to a build failure, with result

[  0% 132/92470] build packages/apps/GmsCore/play-services-co...d/outputs/apk/release/play-services-core-release-unsigned.apk
FAILED: packages/apps/GmsCore/play-services-core/build/outputs/apk/release/play-services-core-release-unsigned.apk 
/bin/bash -c "(rm -Rf packages/apps/GmsCore/play-services-core/build ) && (mkdir -p /home/pants/dev/lineage-microg-docker-volumes/src/LINEAGE_16_0/out/target/common/obj/APPS/GmsCore_intermediates ) && (ln -s /home/pants/dev/lineage-microg-docker-volumes/src/LINEAGE_16_0/out/target/common/obj/APPS/GmsCore_intermediates packages/apps/GmsCore/play-services-core/build ) && (echo \"sdk.dir=/opt/android-sdk\" > packages/apps/GmsCore/local.properties ) && (cd packages/apps/GmsCore && git submodule update --recursive --init ) && (cd packages/apps/GmsCore/play-services-core && JAVA_TOOL_OPTIONS=\" -Dfile.encoding=UTF8\" ../gradlew assembleRelease )"
Picked up JAVA_TOOL_OPTIONS:  -Dfile.encoding=UTF8

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':microg-ui-tools'.
> java.lang.NullPointerException (no error message)

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s

It would be nice if the build system were updated.

Start with packages/apps/GmsCore/Android.mk, the Gradle scripts under packages/apps/GmsCore, and the microg-ui-tools project referenced by the failure. Run packages/apps/GmsCore/gradlew assembleRelease with the Java 9 environment to reproduce the configuration error. Done means the Android Pie build completes without the NullPointerException.