Add test for decoding ID3 PRIV frame

I wrote this whilst investigating a user issue. It looks like
we don't need to do anything about the issue itself, but since
I wrote the test we may as well keep it!

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143549697

library/src/androidTest/java/com/google/android/exoplayer2/metadata/id3/Id3DecoderTest.java

@@ -63,4 +63,15 @@ public class Id3DecoderTest extends TestCase {
     assertEquals("Hello World", textInformationFrame.description);
     assertEquals("Hello World", textInformationFrame.description);
   }
   }
 
 
+  public void testDecodePrivFrame() throws MetadataDecoderException {
+    byte[] rawId3 = new byte[] {73, 68, 51, 4, 0, 0, 0, 0, 0, 19, 80, 82, 73, 86, 0, 0, 0, 9, 0, 0,
+        116, 101, 115, 116, 0, 1, 2, 3, 4};
+    Id3Decoder decoder = new Id3Decoder();
+    Metadata metadata = decoder.decode(rawId3, rawId3.length);
+    assertEquals(1, metadata.length());
+    PrivFrame privFrame = (PrivFrame) metadata.get(0);
+    assertEquals("test", privFrame.owner);
+    MoreAsserts.assertEquals(new byte[] {1, 2, 3, 4}, privFrame.privateData);
+  }
+
 }
 }