DO NOT MERGE: Revert Move startInput for WINDOW_FOCUS_GAIN to background thread

Reason for revert:
Caused an unexpected regression Bug 144174015

Bug: 139806621
Bug: 144103599
Fix: 144174015
Test: Manually verified Bug 144174015 disappeared as follows
 1. Open Gmail then start composing an email
 2. Swipe up the home button to recents then re-launch Gmail
 3. Do the step 2 several times.
 4. Make sure that you can still type something on Gmail.
Change-Id: I9265f01ed2f6e4aca7728d278f06ceea5633dac5