healthd: Check if battery device exists instead of charger drivers before

setting fake properties.

healthd used to check for registered charger drivers to determine if the
device is an always-plugged device with no battery. This patch changes
it to check for battery instead.

Change-Id: Ie4ff5d3f21dc1a83a780a41eb0c929f66b86d631
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>