Integrate new power connnect/disconnect broadcasts

Two new broadcasts, Intent.ACTION_POWER_CONNECTED and
Intent.ACTION_POWER_CONNECTED, that are issued when the device is plugged
and unplugged from USB or AC power.  This pulls two changes from the
open-source Gerrit repo into the internal Donut codeline:

1241 fda6fae1 Added broadcasts for external power events.
9491 37f8ca1e Fix system service crash when booting while on battery power

The current.xml API description has also been updated to include the
new Intent fields; the new API was approved in the original OSS change.