Make lock task check if focused better

Previously using mFocusedActivity, which doesn't seem to update
until after touch (or something).  This now uses
StackSupervisor.getFocusedStack(), which seems to be working ok.

Bug: 17469165
Change-Id: Id06a27addeac1576a71fbc0d8ade1bd87e97b0cb
1 file changed