Move /webrtc/api/android files to /webrtc/sdk/android

I decided to make one webrtc/sdk/android/BUILD.gn for both tests and Java/jni src.

External dependencies needs to be updated after this CL.

Future work is required to clean up the Android api and move
implementation details to /webrtc/sdk/android/src.

BUG=webrtc:5882,webrtc:6804
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2547483003
Cr-Commit-Position: refs/heads/master@{#15443}

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

@@ -0,0 +1,17 @@
+/*
+ *  Copyright 2013 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+package org.webrtc;
+
+/** Interface for observing Stats reports (see webrtc::StatsObservers). */
+public interface StatsObserver {
+  /** Called when the reports are ready.*/
+  public void onComplete(StatsReport[] reports);
+}