Make gesture focal point configurable

[tobrun]

tldr we need to expose a user configurable focal point. A focal represents the x,y value on which a gesture works. In normal cases the x,y from the MotionEvent is used. We need to make this configurable

Currently we already expose an API that works similar to a focal point implementation. When you enable the tracking mode on MyLocationView you also change the focal point to follow that view. An example how that now looks:

                    // Single finger double tap
                    if (mMapboxMap.getTrackingSettings().isLocationTrackingDisabled()) {
                        // Zoom in on gesture
                        zoom(true, e.getX(), e.getY());
                    } else {
                        // Zoom in on user location view
                        // getCenterX also takes in account padding + insets
                        zoom(true, mMyLocationView.getCenterX(), mMyLocationView.getCenterY());
                    }

To be able to support all different use-cases users are requesting we need to expose an API as:

mapboxMap.getUiSettings().setFocalPoint(x,y);

People will mostly use this to center the focal point

mapboxMap.getUiSettings().setFocalPoint(mapView.getWidth()/2,mapView.getHeight()2));

It would be nice to be able to limit if-else in the gesture code above and use a factory pattern to provide an implementation instead.

[1ec5]

A common use case would be to focus zooming on the selected marker. In fact, I suspect that would account for the majority of requests for this feature.

[1ec5]

For the iOS SDK, a configurable gesture anchor point was introduced in #5302. It affects all gestures except for panning.

[tobrun]

Here you can see the gesture in action:

[bleege]

Squashed, Rebased, and Merged into release-android-v4.1.0.