[native] Add jcenter() as a fallback in our react-native-ffmpeg patch

Summary:
This resolves ENG-8959, an issue where our Android build is failing down to intermittent downtime from gradlePluginPortal() preventing us from downloading artifacts necessary for react-native-ffmpeg.

gradlePluginPortal() is a mirror of jcenter(). We switched from jcenter() to it due to intermittent JCenter downtime in D5510. We later reintroduced jcenter() as a fallback for our main native/android/build.gradle Gradle file in D5551, but we failed to reintroduce the jcenter() fallback for our react-native-ffmpeg patch.

This diff introduces the jcenter() fallback for our react-native-ffmpeg patch.

Test Plan:

1. On my local machine, the Android build got past the part that was failing
2. I will make sure Buildkite CI passes before landing

Reviewers: will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12992