KeyguardManager: add lock set and validation methods

The new methods are specifically for non-handset devices
and are guarded by SET_INITIAL_LOCK permission.

Bug: 143612363
Test: TreeHugger
Change-Id: Ifbe6a1b11f4849ec0ea9515ac7b018bb446799ea
Signed-off-by: Julius D'souza <jdsouza@google.com>