Align the behaviour of different lock task modes

Currently lock task modes started by the activity flag
android:lockTaskMode behave differently from those started using
startLockTask(). With those changes lock tasks initiated by non-priv
apps cannot finish without calling into stopLockTask. Revoking the
whitelisting on a locked task will also kill that task, independently
of the way the lock task mode was started.

Bug: 21608206
Change-Id: I841abf1103855e2d7218a4a8ca9b43c105630dc9