Allow setting of custom Location Engine

[cammace]

Currently, it doesn't appear to be a way to use a custom location engine rather then the Lost location provider.

cc: @mapbox/android

[Etienne-io]

Is this issue mean that it will be possible to use MyLocationView without activate the LocationEngine ?

[zugaldia]

@emerciercontexeo It means that you can provide your own LocationEngine implementation, which can use another location provider, or do nothing at all, depending on your app needs.

You can see two examples in this test app. GoogleLocationEngine.java uses Google Play Services, while MockLocationEngine.java simply mocks locations and doesn't do any real location tracking.