[51/n] Generic Repository definition

Create the GenericRepository abstraction with the
MemoryRepositoryImpl implementation and related tests
and test utilities (fakes).

Flag: com.android.window.flags.app_compat_refactoring
Bug: 309593314
Test: atest WMShellUnitTests:GenericRepositoryTest
Test: atest WMShellUnitTests:MemoryRepositoryImplTest

Change-Id: Ic871bea1e033a5fad3e2f3d8674d44495f22c0fb