fix(access-token): match iOS interface for getAccessToken() on Android

[bllanos]

This pull request addresses issue #937

I replicated the behaviour of getAccessToken() on iOS in the native Android code. Both iOS and Android versions now return a promise that either resolves to a string, or is rejected if the access token is null. Previously, the Android version returned a promise that resolved to an object with an accessToken property having the value of the access token.

This is a breaking change, changing the interface of getAccessToken() on Android.

Testing performed

I completed manual testing from a basic app similar to the getting started example, not knowing how or if I should add tests for native Android code:

• getAccessToken() returns a string equal to the access token set previously through setAccessToken().
• getAccessToken() rejects the promise with an error message if null was previously set as the access token through setAccessToken().

I tested the basic app on an Android emulator (API level 28) and on a physical device (Google Pixel, Android 10 build number QP1A.191005.007.A3). The client app uses React Native Version 0.62.2.

I do not have an iOS development environment to test with.

[ferdicus]

Looks good to me 👍

[ferdicus]

@mfazekas, any objections?
I would open a PR with additional docs for MapboxGL methods, and we can tag it as a breaking change for the upcoming release?

[ferdicus]

Added docs in https://github.com/react-native-mapbox-gl/maps/pull/965

[mfazekas]

@ferdicus we should document in the changelog i think that is enough.

I think there could be argument made that it's a breaking change, but it could be considered a bug fix as well. I'd vote for the bug fix.