Split getImeTarget into separate methods

This splits the getImeTarget method into separate getImeLayeringTarget
and getImeControlTarget, to make the usages more explicit. This also
allows maintaining the WindowState type of the IME layering target,
removing the unchecked typecast when used.

Flag: EXEMPT cleanup
Bug: 281029564
Test: atest DisplayContentTests SizeCompatTests WindowStateTests
Change-Id: I636a52ebca1db20caf10e12e618225ebccf93a27