[android] - add unit test for Mapbox

[tobrun]

This PR adds a java unit test for Mapbox and tests the following:

• accessToken validation
• enforcing applicationContext
• switching between Android Connectivity change events and manually managing them

Code coverage after adding the test:

[mention-bot]

@tobrun, thanks for your PR! By analyzing this pull request, we identified @zugaldia to be potential reviewers.

[zugaldia]

field.set(mapbox, mapbox); got me confused, thanks for clarifying, maybe adding a comment before that line explaining that's required because it's a singleton?