Make Raw Resource Scheme to be public, accessible from outside.

library/core/src/main/java/com/google/android/exoplayer2/upstream/RawResourceDataSource.java

@@ -58,7 +58,7 @@ public final class RawResourceDataSource implements DataSource {
     return Uri.parse(RAW_RESOURCE_SCHEME + ":///" + rawResourceId);
   }
 
-  private static final String RAW_RESOURCE_SCHEME = "rawresource";
+  public static final String RAW_RESOURCE_SCHEME = "rawresource";
 
   private final Resources resources;
   private final TransferListener<? super RawResourceDataSource> listener;