Add a test API allowing overriding the GameService provider.

This is so that CTS tests can be run against a test GameService
rather than against whatever GameService a particular device
has set as its default.

Ignore-AOSP-First: GameService incomplete in AOSP
Test: atest CtsGameServiceTestCases GameServiceProviderSelectorImplTest
Bug: 202417555
Bug: 206128693
Change-Id: I5d0d1a4047949117850818c14b5df617314efcf5