Do not cleanup IME window if show is requested.

This CL fixes a race condition for IMM#showSoftInput, which surfaces
when it's called during an IME hide animation.
IMM#showSoftInput ends 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 think a clean fix is to have IMM#showSoftInput call WIC#show
synchronously. However, this requires a significant refactoring.

As a short term fix, this CL adds a boolean field indicating whether or
not IMM#showSoftInput has been called. If it's called, we skip calling
IMM#notifyImeHidden.

Bug: 221483132
Bug: 225674038
Test: atest InputMethodStressTest
Test: atest CtsInputMethodTestCases
Test: atest WindowInsetsAnimationControllerTests
Change-Id: I36d570630085d0bc34097a2433208601dc9cb0fd