Make GameManagerService JNI registration lazy

This service is optional. As such, make its JNI registration lazy,
supporting build-time removal. Also refactor service test JNI libs
to support related unit tests.

Bug: 375264322
Flag: EXEMPT refactor
Test: m + boot test
Test: atest CtsGameManagerTestCases GameManagerServiceTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:756f75ebd5fd493ac6b804001e49ab97d8350e34)
Merged-In: Idee4cbd973e7c09d435273f927bd787fc2afd0d6
Change-Id: Idee4cbd973e7c09d435273f927bd787fc2afd0d6