Fix flickers when switching bubbles with IME open

When switching bubbles while the IME is open, we now perform the
switch after the IME is fully hidden.

Flag: EXEMPT bug fix
Fixes: 384838831
Test: atest BubbleStackViewTest
Test: manual
       - have 2 floating bubbles
       - expand one and request IME
       - select the second bubble
       - observe switching does not flicker
       - repeat switching
Change-Id: If269ccf55ace7acec4b8b10b723c257315aec120