Lock top task if whitelisting arrives after start.

If the DevicePolicyManagerService updates the whitelist after a task
in the whitelist has started then the task won't have started locked.

When the updated whitelist arrives this change automatically locks the
topmost task if it is in the whitelist.

Also more locktask debugging.

Fixes bug 21031298.

Change-Id: I2494af6f2819ca91bc01abc5decb3d1adc088226