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
Change-Id: Idee4cbd973e7c09d435273f927bd787fc2afd0d6