[android] Offline event

[zugaldia]

This PR adds a new event to the SDK in the offline manager to help inform offline usage. Please note that it's still a WIP - not all fields are finalized, and tests are missing.

mapbox/mapbox-events-android#193 is required for this PR to land (and build).

/cc: @mapbox/maps-android

[tobrun]

nit: rename to onCreateOfflineRegion, Map is not a relevant keyword in this context. A user can also create an offline region without a map.

[osana]

@captainbarbosa I am wondering if there is a difference between iOS and Android in this data. For each OfflineDefinition that we track. I have:

OfflineGeometryRegionDefinition :
  private String styleURL;
  private Geometry geometry;
  private double minZoom;
  private double maxZoom;
  private float pixelRatio;

and

OfflineTilePyramidRegionDefinition:
  private String styleURL;
  private LatLngBounds bounds;
  private double minZoom;
  private double maxZoom;
  private float pixelRatio; 

Not sure what could be viewed as a source ?

[captainbarbosa]

The source was meant to represent the list of sources that belong to a style. Both OfflineGeometryRegionDefinition and OfflineTilePyramidRegionDefinition currently don't have access to a style's sources as you would normally retrieve them from calling mapboxMap.getSources(). This limitation also exists on iOS. Its possible we can pass over the sources from mbgl so we can access them and pass them into the event.