Add lockTaskOnLaunch attribute.

The new AndroidManifest activity attribute, lockTaskOnLaunch attribute
is a boolean that puts the system in lockTask mode when true and if
the activity specified is the root of a privileged task.

This bug also fixes lockTask mode for root activities that finish
themselves. Previously finish was not allowed even if there were
activities left in the task that were still valid.

A NullPointerException for lock task toasts has also been fixed.

Fixes bug 19995702.

Change-Id: Iba6976b1a0cc5a22eb526db66d2e9af66445541f