Android: Use scoped java refs

We currently use raw jobject in our code mixed with sporadic
ScopedLocalRefFrame. This CL moves every jobject into a scoped object,
either local, global, or a parameter. Also, this CL uses the JNI
generation script to generate declaration stubs for the Java->C++
functions so that it no longer becomes possible to mistype them
without getting compilation errors.

TBR=brandt@webrtc.org

Bug: webrtc:8278,webrtc:6969
Change-Id: Ic7bac74a89c11180177d65041086d7db1cdfb516
Reviewed-on: https://webrtc-review.googlesource.com/34655
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21387}

sdk/android/src/java/org/webrtc/NV21Buffer.java

@@ -12,6 +12,7 @@ package org.webrtc;
 
 import java.nio.ByteBuffer;
 
+@JNINamespace("webrtc::jni")
 public class NV21Buffer implements VideoFrame.Buffer {
   private final byte[] data;
   private final int width;