Support usage of images already loaded in map style for location layer drawables

[cammace]

Pointed out in #183 (comment), it makes sense to also allow users to use the bitmap id which defines the sprite inside the map style for their location layer icons.

[DejanRistic]

@cammace is there a ETA for getting this into the snaphsot build? We are looking to migrate over to mapbox 6.x so we aren't to behind, but can't make the switch until the location plugin has support for this as we would lose a feature without it. And 6.x sort of requires us to move to the location plugin, I guess i could fork the plugin and expose the methods for the short term.

Just curious, Thanks!