feat(android): allow building exoplayer from source (#3932)

* feat(android): allow to build from source code

* fix(android): allow to build plugin from source

examples/basic/android/build.gradle

@@ -16,6 +16,9 @@ buildscript {
         // useExoplayerSmoothStreaming = false
         // useExoplayerDash = false
         // useExoplayerHls = false
+
+        // uncomment this line to be able to build from media3 source code
+        // buildFromMedia3Source = true
     }
     repositories {
         google()