Use BatteryManagerInternal to determine low battery.

BatteryManagerInternal calls into BatteryService, which has its own
logic for identifying low battery. BatteryService's low battery logic
takes a user's setting into account. Switch to using BatteryService's
logic so we can standardize on battery low logic.

Bug: N/A
Test: Android builds
Change-Id: I0cc4538bb80ce21e42721eccf6b6671d52a24084