Don't set return to recents on every resume

Checking for previous task to be the Recents activity every time
through resumeTopActivityLocked() leads to missetting the
mTaskToReturnTo to Recents for Recents. Instead check for
Recents launching a task in moveTaskToFront and startActivity.

Fixes bug 15832448.

Change-Id: Ib64fe26f3fd3fdcd878edafb041928ec02757e63