Return slices of ByteBuffers from getDataY/U/V() in I420Buffers.

I420Buffer implementations (I420BufferImpl and WrappedNativeI420Buffer) rely on
the 'position' of the underlying ByteBuffers to indicate the start of Y, U, and
V channels.  Returning slices prevents callers from altering the state of the
I420Buffer by changing the position.

ByteBuffers are especially prone to accidentally moving the position: relative
read operations (such as get()) increment the position by the size of data read.

BUG=webrtc:8303

Change-Id: I52edce8a3bf46a6c41980ff5110a9480f021f22f
Reviewed-on: https://webrtc-review.googlesource.com/4521
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20050}

sdk/android/src/java/org/webrtc/WrappedNativeI420Buffer.java

@@ -53,17 +53,20 @@ class WrappedNativeI420Buffer implements VideoFrame.I420Buffer {
 
   @Override
   public ByteBuffer getDataY() {
-    return dataY;
+    // Return a slice to prevent relative reads from changing the position.
+    return dataY.slice();
   }
 
   @Override
   public ByteBuffer getDataU() {
-    return dataU;
+    // Return a slice to prevent relative reads from changing the position.
+    return dataU.slice();
   }
 
   @Override
   public ByteBuffer getDataV() {
-    return dataV;
+    // Return a slice to prevent relative reads from changing the position.
+    return dataV.slice();
   }
 
   @Override