Fix gmail flicker issue in split screen

This issue is caused by surface invisible before transition.
And this symptom caused by we got wrong resume callback by another
side split activity.

Fix this by change prioirty in TaskFragment#resumeTopActivity.
We should check non activities are pausing before checking
top activity resumed.

Fix: 257008620
Test: manual
Test: pass existing tests
Change-Id: I6ef987bf694678b8422f3e7526944ee9004c61e1