Cleanup IME layering/input/control target fields

This is a general cleanup of all the references to IME layering target,
IME input target and IME control target. This also adds some
documentation to these fields and the methods which update and refer to
them.

Previously we were refering to the IME layering target as simply the IME
target, which was sometimes confusing. This makes the reference
explicit.

Flag: EXEMPT cleanup
Bug: 281029564
Test: atest DisplayContentTests DualDisplayAreaGroupPolicyTest
  InsetsStateControllerTest WindowStateTests ZOrderingTests
Change-Id: I7d224a06d8d55e605c37b119860c3c82a75d97f3