Add a TODO to enforce ParsableByteArray.limit

I was trying to understand why this isn't always checked and didn't
realise there was already a bug tracking this - this way a future
confused reader knows something isn't working quite as intended.

PiperOrigin-RevId: 313765935

library/common/src/main/java/com/google/android/exoplayer2/util/ParsableByteArray.java

@@ -29,6 +29,8 @@ public final class ParsableByteArray {
   public byte[] data;
 
   private int position;
+
+  // TODO(internal b/147657250): Enforce this limit on all read methods.
   private int limit;
 
   /** Creates a new instance that initially has no backing data. */