Add unit tests for hiding the IME nav bar

This adds unit tests for the newly added IME captio bar insetsSource,
and for the code paths through InsetsController for showing and hiding
the IME nav bar.

Bug: 310199730
Test: atest
  FrameworksImeTests:InputMethodServiceTest#testShowHideImeNavigationBar_doesDrawImeNavBar
  FrameworksImeTests:InputMethodServiceTest#testShowHideImeNavigationBar_doesNotDrawImeNavBar
  InsetsSourceTest#testCalculateInsets_imeCaptionBar
Change-Id: I84accc9666eef2bdeaa7073acd7f364938770b65