Add unit tests for InputMethodManagerService

Injecting and mocking dependencies to InputMethodManagerService to make
it unit testable.

Add InputMethodManagerServiceTestBase base class to help unit test IMMS.

Add the first batch of unit tests in InputMethodManagerServiceWindowGainedFocusTest to cover the behavior of IMMS#startInputOrWindowGainedFocus(), which should help us catch any regressions caused by future changes to the code.

Bug: 240359838
Test: atest com.android.server.inputmethod.InputMethodManagerServiceWindowGainedFocusTest
Change-Id: Id61087f1e176738910f5acfe912f8682f727255c