Clean up the lock task APIs for COSU devices.

Clean up and increase readability of internal handling of lock task mode APIs.
Add a public API to query the lock task mode state with pinned and locked as
possible outcomes. Additionally, change wording in lock task toasts when in
locked mode and update the javadoc regarding onLockTaskModeEntering and
onLockTaskModeExiting to represent the actual behaviour.

Bug: 19377096
Change-Id: Ia563078ca6ef6d6fc7e75130e6b94ba18af69340