Remove testing variant of setImeLayeringTarget

This removes the testing variant of setImeLayeringTarget, that was
simply updating the field without reporting the update to other
components. Tests are updated to use the real method, which triggers all
the updates. Mentioned below are the tests with slight changes to
accomdate this.

The two WindowState tests were using old code paths, and were not
setting the IME control target properly. They are now updated in line
with recent code changes.

Flag: EXEMPT cleanup
Bug: 281029564
Test: atest DisplayContentTests#testComputeImeLayeringTarget_shouldNotCheckOutdatedImeLayeringTargetWhenRemoved
  WindowStateTests#testAdjustImeInsetsVisibilityWhenSwitchingApps
  WindowStateTests#testAdjustImeInsetsVisibilityWhenSwitchingApps_toAppInMultiWindowMode
Change-Id: Ic7f336e578e397d07f6bae583de81b15450b2ede