If the 0x1000 Unified Battery Level Status feature exists, expose the battery
level.

The main drawback is that while a device is plugged in its battery level is 0.
To avoid exposing that as 0% charge we make up a number based on the charging
status.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>