IME: make WindowInsetsController.show(ime()) more robust

We need to make sure the served view is up-to-date, otherwise we might
erroneously reject or allow a request to show the IME via WindowInsetsController.

Fixes: 190217891
Test: atest WindowInsetsControllerTests#testShowIme_immediatelyAfterDetachAndReattach
Change-Id: Ib41412d5d5636bdc236aa94c4ee4702686b6d10f