[Android] - Disable mapbox access token

[sjoerdhuininga]

At the moment it seems the Android SDK will crash when you provide an invalid access token (defined as it being NULL or not starting with either PK or SK). Since we're not using Mapbox but our own tileset we don't really need a Mapbox token.

Current behaviour
App crashes when not entering a Mapbox token

Expected behaviour
App doesn't crash when leaving an empty Mapbox token

[cdiaz]

I have same problem

[petr-pokorny-1]

There is a PR #90 which addresses this problem, after it will be merged and released developer will be able to use Mapbox.getInstance(Context context) overload without access token.