Add proguard configuration for Cast extension

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191422866

extensions/cast/build.gradle

@@ -21,6 +21,7 @@ android {
     defaultConfig {
         minSdkVersion 14
         targetSdkVersion project.ext.targetSdkVersion
+        consumerProguardFiles 'proguard-rules.txt'
     }
 }
 

extensions/cast/proguard-rules.txt

@@ -0,0 +1,4 @@
+# Proguard rules specific to the Cast extension.
+
+# DefaultCastOptionsProvider is commonly referred to only by the app's manifest.
+-keep class com.google.android.exoplayer2.ext.cast.DefaultCastOptionsProvider