Android: Annotate and generate JNI code for VideoFrame.java

This CL also merged native_handle_impl.cc and videoframe_jni.cc to keep
all JNI code for the same Java class in the same file, and also renames
this file to jni/videoframe.cc.

The classes AndroidVideoBufferFactory and JavaVideoFrameFactory are
now unnecessary since we cache everything and can be simplified to
global static functions instead.

Bug: webrtc:8278
Change-Id: I03d7b0bbde64cfb407cd6210478ddf9d5599cd8c
Reviewed-on: https://webrtc-review.googlesource.com/22923
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20677}

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

@@ -21,4 +21,9 @@ import java.lang.annotation.Target;
  */
 @Target(ElementType.METHOD)
 @Retention(RetentionPolicy.CLASS)
-@interface CalledByNative {}
+@interface CalledByNative {
+  /*
+   *  If present, tells which inner class the method belongs to.
+   */
+  public String value() default "";
+}