Adds logging to push settings changed to statsd.

Introduces atom and logging for every time a setting is updated.
We also include the previous value to validate our logging
data quality. We need to add the INTERACT_ACROSS_USERS_FULL
permission to prevent a security exception during testing; since the
logging occurs from system_server uid, this security exception should
not occur normally.

Test: Manual by inspecting the statsd output while toggling settings.
Change-Id: I92ca2afa596e54e61936e22611d749bb60ccb364