Merge ImeTargetChangeListener to InputMethodManagerInternal

This CL reworks how WMS tells IMMS about visibility changes on IME
targets windows.

With this CL we simply start using InputMethodManagerInternal.

Main motivations are:

 * We can see all the inter-component callbacks at one place in IMMS.
 * This removes one this-escape from the constructor.
 * No need to change WMS when we start relying on multiple instances
   of ImeVisibilityStateComputer.

At this momemt there must be no observable behavior change.

Fix: 356876005
Test: atest WmTests:WindowStateTests
Test: atest FrameworksInputMethodSystemServerTests
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Ia711026604cd2e4588912fe2aa8bbd69808249aa