Hide the IME using InputMethodManagerInternal, because doing it that way actually works.

This technique is also used by the Recents animation to hide the IME. It replaces the AV back press workaround entirely, which fixes the IME hanging around issue (and by extension, the weirdness around the insets).

Test: open a bubble (go/use-bubbles), focus an input field, then tap outside the bubbles UI to close the stack, observe that the IME hides
Test: open a bubble, focus an input field, switch to another bubble, observe IME hides
Test: open bubble, focus input field, switch to another bubble, focus input field, switch to the original bubble, focus input field, collapse stack, observe IME hides (previously the IME would remain visible)
Bug: 159369847
Bug: 159561847
Change-Id: I09f27b3c187a7bcfbef663cd3101a74ad8ab95ab