Clear last paused activity.

Moving an activity to the back used to move it behind the launcher.
This meant that the test in resumeTopActivityLocked() for
mLastPausedActivity being equal to the top activity would fail after
moveTaskToBack(). In the new activity stack model the launcher is
on a different stack so if there is only one task then the test
now passes. Clearing mLastPausedActivity when moving an activity
forward now passes the test.

Fixes bug 10022212.

Change-Id: I17932d5c87d2483f13c29ae85a7067202a08f34e