Fix Cast demo app's blaze build and enable proguarding

PiperOrigin-RevId: 233372668

demos/cast/src/main/AndroidManifest.xml

@@ -17,6 +17,8 @@
     package="com.google.android.exoplayer2.castdemo">
 
   <uses-permission android:name="android.permission.INTERNET"/>
+  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
+
   <uses-sdk/>
 
   <application android:label="@string/application_name" android:icon="@mipmap/ic_launcher"

extensions/cast/proguard-rules.txt

@@ -2,3 +2,9 @@
 
 # DefaultCastOptionsProvider is commonly referred to only by the app's manifest.
 -keep class com.google.android.exoplayer2.ext.cast.DefaultCastOptionsProvider
+
+# ExoCastOptionsProvider is commonly referred to only by the app's manifest.
+-keep class com.google.android.exoplayer2.ext.cast.ExoCastOptionsProvider
+
+# Don't warn about gms
+-dontwarn com.google.android.gms.**