Update Gradle plugin to 3.3.0

[aforcier]

As of this writing, 3.3.0 is the latest gradle plugin. We should update the project to use it. However, at the moment updating generates some build warnings.

The first warning, WARNING: API 'variant.getGenerateBuildConfig' is obsolete, can be resolved by using the new provider config for this Crashlytics task:

android.applicationVariants.all { variant ->
    variant.generateBuildConfig.dependsOn(generateCrashlyticsConfig)
}

to

android.applicationVariants.all { variant ->
    variant.generateBuildConfigProvider.configure {
        dependsOn(generateCrashlyticsConfig)
    }
}

The next warning requires the Fabric gradle plugin to be updated to support 3.3.0 and fix its internal warnings - we should keep an eye on this changelog: https://docs.fabric.io/android/changelog.html#fabric-gradle-plugin, then update the version we use along with our Gradle version.

[aforcier]

Note: The gradle version has already updated to 3.3.0, but this issue now tracks what we need to do to silence the obsolete warnings we're getting when building.

[aforcier]

Fixed in #911.