Use the underlying type of Java Video Buffer on Java -> C++ Frame Buffer

Just like the C++ API, add a method in Java VideoFrame.Buffer that
describes the underlying implementation.
Use this method to properly select AndroidVideoBuffer
or AndroidVideoI420Buffer in Java -> C++ Video Frame Conversion.

Also, add a test case for WrappedNativeI420Buffer
in VideoFrameBufferTest for consistency.

Bug: webrtc:12602
Change-Id: I4c0444e8af6f6a1109bc514e7ab6c2214f1f6d60
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/223080
Commit-Queue: Byoungchan Lee <daniel.l@hpcnt.com>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Xavier Lepaul‎ <xalep@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34545}

sdk/android/api/org/webrtc/VideoFrame.java

@@ -34,6 +34,15 @@ public class VideoFrame implements RefCounted {
    * and the buffer needs to be returned to the VideoSource as soon as all references are gone.
    */
   public interface Buffer extends RefCounted {
+    /**
+     * Representation of the underlying buffer. Currently, only NATIVE and I420 are supported.
+     */
+    @CalledByNative("Buffer")
+    @VideoFrameBufferType
+    default int getBufferType() {
+      return VideoFrameBufferType.NATIVE;
+    }
+
     /**
      * Resolution of the buffer in pixels.
      */
@@ -63,6 +72,11 @@ public class VideoFrame implements RefCounted {
    * Interface for I420 buffers.
    */
   public interface I420Buffer extends Buffer {
+    @Override
+    default int getBufferType() {
+      return VideoFrameBufferType.I420;
+    }
+
     /**
      * Returns a direct ByteBuffer containing Y-plane data. The buffer capacity is at least
      * getStrideY() * getHeight() bytes. The position of the returned buffer is ignored and must