Extend clearing of mLastPausedActivity.

Add more locations where mLastPausedActivity must be set null before
calling resumeTopActivityLocked().

Also separate the "No History" activity from mLastPausedActivity by
adding mLastNoHistoryActivity. This is necessary because now
mLastPausedActivity will be null when mLastNoHistoryActivity
is checked.

Continues to fix bug 10022212.

Change-Id: Ida543b0638c865127691dc51a6e0635062539659