Update Android Mapbox plugins and gradle

[m0nac0]

I upgraded the Mapbox sdk and the two Mapbox plugins we are using to the latest version, as well as gradle, because the older gradle version didn't play nicely with the new Mapbox plugins.
This should fix #263 and hopefully also #248
Big thanks to @AAverin for keeping track of the fix upstream.

[m0nac0]

I changed the gradle version to 3.5.0, which is still an upgrade from what we had before this PR and enough to make it compile. Gradle 3.6.1 causes an issue with the CI because it requires an Android NDK version that is currently not preinstalled on the Github Action runners.
The alternative would have been to install the right version of the NDK on each run of the CI, but this would have added approx. 1.5 minutes to the build time, so I decided to stick with this solution for now.

[AAverin]

@m0nac0 any way I can help to make it a release? I a bit blocked by the map crashing

[m0nac0]

@AAverin I'm sorry for the delay, but merging will be blocked until one of the other contributors approves the PR. If you depend on this, I'd recommend you to use a git dependency with the ref tag in pubspec.yaml and depend on the m0-update-sdk branch I used for this PR. There's an explanation of this at https://dart.dev/tools/pub/dependencies#git-packages.