Lock task doesn't always end properly.

This can result in the keyguard not being re-enabled, the status
bar being in a bad state, and various other weirdness.  When a task
is going away, it needs to send the standard lock task ending
messages so that exiting the state can be handled properly.

Bug: 16010327
Change-Id: Id7ee632f4765af0af209b70fb69dd75f2e541820