[Gradle] Replace jcenter() with gradlePluginPortal()

Summary:
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 of react-native-ffmpeg.

Test Plan: CI and tested locally

Reviewers: varun, ginsu, rohan, ashoat, tomek

Reviewed By: ashoat

Subscribers: abosh

Differential Revision: https://phab.comm.dev/D5510