Set focused app when top activity is resumed

In a multi-resumed environment, like in a freeform device, the top
activity can be resumed but it might not be the focused app. This
causes some failures in Pip tests. To fix it, the top activity can
be explicitly set to the focused app in TaskFragment.resumeTopActivity()
function.

Bug: 194224954
Test: atest PinnedStackTests#testEnterPictureInPictureMode

Change-Id: I646c9950e74c589c155a32a787b5165598cecb61