We're having issues with jCenter being down/unavailable (see: https://status.gradle.com/incidents/c65zmfw32lpm) which affects at least `react-native-video` and `react-native-ffmpeg`.
In this diff we:
A. Replace `jcenter()` with `gradlePluginPortal()` (which is a jCenter mirror provided by Gradle) in `native/android/build.gradle`. This resolves issues with `react-native-video`.
B. Patch `react-native-ffmpeg` to replace `jcenter()` with `gradlePluginPortal()` in order to properly resolve transitive dependencies.