Fix 3388705: Explicitly check for low battery level

This fixes a bug in StatusView believed to be caused by
seeing "invalid charger" update from BatteryService.

The code normally relies on "interesting events", as determined
by KeyguardUpdateMonitor.  I believe something else is
triggering an update (perhaps a SimStateChanged event) that updates
the status without also updating StatusView.mShowingBatteryInfo
and mPluggedIn.

The safer way to do this is to explicitly check the battery
level before telling the user the device needs to be charged.

Change-Id: Ic39ed86c78a157dc9fbdef4d76a9c3db39ccafca