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