Adding Firebase crashlytics

[langsmith]

Resolves #625

[langsmith]

Phew, finally ready for review when ya'll get a chance.

It's no longer in the actual pr, but during testing, I set up forcing a test crash as documented at https://firebase.google.com/docs/crashlytics/force-a-crash.

After linking the debug version from Fabric to Firebase, the crashes started to come into the Firebase console.

Couple of things to know about this pr:

1. Dependency management got kinda' wild while dealing with new/upgraded Firebase libraries. As the error messages continued one after another, it became clear that migrating to AndroidX dependencies would be the best choice for dealing with these conflicts and for the app in general moving forward. Thanks in large part to Android Studio's automatic AndroidX migration tool, this pr transitions the app to AndroidX. Most of this pr's changed classes are because of importing new AndroidX packages. Use the file filter if you want to a see a more manageable overview of this pr's changes (https://github.com/mapbox/mapbox-android-demo/pull/1104/files?file-filters%5B%5D=.gradle&file-filters%5B%5D=.yml&file-filters%5B%5D=dotfile excludes .java/.kt/XML files)
2. The Firebase setup required me to bump the minSDK to 16. I became comfortable with this new minSDK requirement after looking at the Platform versions breakdown on https://developer.android.com/about/dashboards. I'm comfortable with excluding the .6% of devices that are pre-16. If you have any objections to bumping this, please let me know.
3. The initializeFirebaseApp() that was in the MapboxApplication class, was removed because the initialization is now done behind the scenes by Firebase's core library.
4. Firebase requires the Google Services plugin and google-services.json file in the MapboxAndroidDemo folder to do the crash reporting. The JSON file holds special keys, so it can't be publically committed to Github. I've added the release flavor's Firebase JSON file text as a CircleCI environment variable and created a circle.yml task to create the file during CI runs.

[tobrun]

Looks great! Main request for change is to optimize circle configuration for external pull request. We don't allow using env variables for them and we shouldn't execute steps that rely on them. See https://github.com/mapbox/mapbox-gl-native/blob/master/circle.yml#L573 for an example.

[langsmith]

Ok @tobrun , I've addressed your feedback

Main request for change is to optimize circle configuration for external pull request. We don't allow using env variables for them and we shouldn't execute steps that rely on them. See https://github.com/mapbox/mapbox-gl-native/blob/master/circle.yml#L573 for an example.

I believe 6e313e0 correctly addresses the key/secret generation in the way you were envisioning. If not, let me know!

[tobrun]