Add persistence of tasks for TaskManager & BatteryController

Tasks are persisted only if the client has the RECEIVE_BOOT_COMPLETED
permission. This applies to both periodic and one-off tasks.
Write out task as xml, using PersistableBundle for the extras data.
Todo: Add persistable bundle to xml when ag/468207 is merged.

Also added BatteryController logic.

Change-Id: I23eeeb8b3bc6ba155e7fa4ec25857a68ee8b1567