Skip IME show when target is changing.

There can be a race condition when IME is requested and IME target is
changing. If Ime target is closing, we will skip showing IME for it.

Fix: 144503054
Test: Manually:
        1. Open chrome, type in omnibox and swipe to home
	2. relaunch chrome and it will request IME
	3. swipe again to home
	4. repeat steps 2 and 3 quickly and verify
	   that IME doesn't hang on launcher

Change-Id: Iec9e44c5eb71eb4c6de693eb68c1887e7f883d1d