Android: Generate JNI code for stats

This CL also unifies the functions for converting from C++ to Java, and
generates the boiler plate for converting C++ vectors to Java arrays.

Bug: webrtc:8278
Change-Id: I262e9162beae8a64ba0e8b6a27e1081207b03961
Reviewed-on: https://webrtc-review.googlesource.com/26020
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20918}

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

@@ -13,5 +13,5 @@ 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);
+  @CalledByNative public void onComplete(StatsReport[] reports);
 }