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