Make GPS and compass resources public to allow for customisation

[ljbade]

In the public resource XML we should allow apps to override these resources:

• compass bitmap
• GPS marker bitmaps (and the colour of the accuracy ring which should be moved to a resource instead of a static value)
• Mapbox logo?
• attribution button

The use case here is say an app wants a pink GPS marker to match their app's theme:

• they copy our GPS bitmap
• edit the colour to pink
• override the resources
• also change the accuracy ring colour

/cc @tobrun

[ljbade]

Also add the default maker to the list to allow changing the colour etc, similar to iOS. Better solution than #2558