Fix a NPE in BatteryStatsImpl

ServiceManager.getService("batteryproperties")) may return null for some
devices right after boot. (We don't know why, need further investigation)
This causes async batterystats update to crash, leaving BatteryStats in a
bad state (OnBattery() == true, but mOnBatteryTimeBase is not running),
which does not accept aggregated stats update anymore.

Bug: 109930230
Test: manual
Change-Id: I0654beff95f0a2b9df2567f1a2efffd3330e58ff