QuickSettings : Battery initial status

Battery tile upon being added relies on a battery level changed call
to update its status. Thus if upon adding, the battery level is not changing
or till the period when it updates, the tile shows an unknown state.

This patchset resolves this issue by making the controller store its last received value
and allow them to be retrieved.

https://jira.cyanogenmod.org/browse/CYAN-1374

Change-Id: Ia2e887e545430ed02c179a3c8f4b7e269a4817c8