Fix unprotected variable access by serializing.

The variables mKeyguardDisabled and mAllowDisableKeyguard were
being modified unprotected by mKeyguardTokenWatcher. Fix is to
serialize accesses to these variables by only referencing them
from the same Handler that mKeyguardTokenWatcher uses. Eliminates
synchronization blocks and mKeyguardDisabled variable.

Fixes bug 7045624.

Change-Id: I6355aa393507408296316bee61e178dc81e2a172