battery: Add the internal Battery API

battery: Add the internal Battery API

This patch adds an API for internal BlueZ code to expose battery
information to org.bluez.Battery1 interface. The motivation behind this
is that there is going to be other places than GATT BAS handler that
exposes battery information, for example internal plugins and the
BatteryProvider1 D-Bus API for external clients.

Reviewed-by: Daniel Winkler <danielwinkler@google.com>
Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>