power: android-battery: push uevent whenever charge source changes

Ensure userspace reads an up-to-date value for charging status whenever the
charge source is updated.  Avoid races where stale charging state may be
reflected in userspace until the next battery state poll.

Change-Id: Ia744db48584e9e9abf442710c279af9e3a25c079
Signed-off-by: Todd Poynor <toddpoynor@google.com>