Add proguard configuration for Cast demo app

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191442704

demos/cast/build.gradle

@@ -29,7 +29,10 @@ android {
         release {
             shrinkResources true
             minifyEnabled true
-            proguardFiles getDefaultProguardFile('proguard-android.txt')
+            proguardFiles = [
+                "proguard-rules.txt",
+                getDefaultProguardFile('proguard-android.txt')
+            ]
         }
         debug {
             jniDebuggable = true

demos/cast/proguard-rules.txt

@@ -0,0 +1,6 @@
+# Proguard rules specific to the Cast demo app.
+
+# Accessed via menu.xml
+-keep class android.support.v7.app.MediaRouteActionProvider {
+  *;
+}