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.