power: android-battery: leave full/not-charging status when charger re-sensed

If charger was connected and a charge source change event occurs such that
the charger is still connected (or a different charge source connected),
do not overwrite an existing state such as full or not-charging.  Only
switch from discharging to charging on a charge source change event.

Change-Id: I1b841c8f3a92bf15074999e18b17d1d19ce028d7
Signed-off-by: Todd Poynor <toddpoynor@google.com>