Check for apps closing and restore mExiting test.

Removal of the mExiting test in a previous CL was a mistake leading
to z-order errors. In particular the auto complete dialog was on top
of the IME and was being dismissed due to touches on the IME.

Restoring mExiting alone missed cases where apps were exiting which
don't set mExiting. Adding a test for membership in mClosingApps
fixes that.

Bug: 7327220 fixed.
Change-Id: I3965b8a07080d1347bdada51ffeafe6ef2e32c8e