Fixed issue with IME displaying on-top of nav bar.

Caused by some recent refactoring. We now make sure the IME
has the higher animation layer in its base layer of the window
it is targeting.
Also, consolidated some of our test functions.

Bug: 32916670
Test: bit FrameworksServicesTests:com.android.server.wm.WindowLayersControllerTests
Change-Id: I0b1abd6fead981cfc810488cc785261abba5341d