Unify handler posting to InsetsController from different methods

Different methods of InputMethodManager call into InsetsController#show/hide. If the view is running on a different thread, the call will be posted on the view handler. As this logic was duplicated in those function, this CL unifies them in one method that is then called.

Bug: 407546107
Test: atest CtsInputMethodTestCases
Flag: EXEMPT refactor
Change-Id: I9836dded7b955690b8ac768fe849654d182346a9