Transfer IME callbacks when app's focused root view changes.

Previously when curRootView changes IMM, the back callbacks are not
moved to the new focused ViewRootImpl. As a result if IME is up during
the focus change, the back callback would fail to unregister when
IME tries to hide itself.

Test: atest InputMethodServiceLifecycleTest
Test: atest CtsInputMethodTestCases:InputMethodServiceTest
Test atest CtsInputMethodTestCases:KeyboardVisibilityControlTest
Bug: 232660571
Bug: 232331013

Change-Id: Id30e51c74afbcce1f22d87af77e8404b4f0ae7d2