Notify IMMS synchronously on IME insets hidden.

Currently, IMMS will be notified asynchronously when an IME hide
animation finishes, via message dispatching through IMS
(IMM#notifyImeHidden -> IMS#notifyImeHidden -> IMMS#hideMySoftInput).
This creates a race condition when IMM#showSoftInput or WIC#show is
called around the end of hide animation.

This CL fixes the race condition by synchronously and directly
calling IMMS#hideSoftInput from IMM#notifyImeHidden.

Note that there is still another race condition for IMM#showSoftInput
(not WIC#show) if it's called during an IME hide animation;
IMM#showSoftInput ended up calling WIC#show asynchronously, but at that
time the running IME hide animation may have already been finished
successfully and WIC#show may fail to cancel the hide animation
(then the cleanup IMM#notifyImeHidden hides the IME again disruptively).
I will fix the latter issue in a separate CL.

Bug: 221483132
Bug: 225674038
Test: atest InputMethodStressTest
Test: atest CtsInputMethodTestCases
Test: atest WindowInsetsAnimationControllerTests
Change-Id: I7c71dc5a1d6b61aa79d1666f0e257e6401e4adb2