power: android-battery: Allow changing current in charging state

When a new charge source is set, enable charging and set the charging
current, unless we've paused charging due to battery full or battery
health problems.

Change-Id: I129e621c455a941e264dc431b4eb9db6a17c9f7b
Signed-off-by: Danke Xie <d.xie@sta.samsung.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>