Reset BatteryStats when too large for clients to handle

Reset BatteryStats when too large for clients to handle

When BatteryStats history is too large, the Settings app crashes.
Reset the battery stats by marking an *OVERFLOW* event at the
beginning of the newly reset history.

This seems to happen in Retail mode when the devices are on charger
for an extended period of time and accumulate a sufficiently large
history. Resetting makes sense because the phone is most likely
charged and will reset anyways when unplugged.
Bug: 32540341
Test: Manual

Change-Id: I8c2892458b5d9ce64b1c44aa2e2a000340e71e63