Make IME force stash the Taskbar.

This change moves the IME flag to be part of FORCE_STASHED rather than
STASHED_IN_APP, since there are cases where we are not in an app but
still want to stash the IME.

You can still bring up transient taskbar over IME since we do not set
the IME flag for transient mode (see TaskbarStashController#shouldStashForIme() first guardrail).

Test: Manual
Flag: N/A
Fix: 336592511
Change-Id: Iafa32b1a52f4bfbc0d3a4e77b19c82149dc42d0a