Bluetooth: MPL: Use set_track_position more excessively

The set_track_position handles both correct clamping of the
position, as well as the notification, so use that instead
of setting the value and sending notifications manually.

Also adds a new function, set_relative_track_position,
to help set relative positions.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>