Deprecate PeerConnectionFactory#createVideoSource(VideoCapturer).

This is done in preparation of moving VideoCapturer out of
video_api_java. Clients should update to using
createVideoSource(boolean).

CapturerObserver is moved to a separate file because it needs to stay
in video_api_java to allow VideoSource to depend on it.

Bug: webrtc:9496
Change-Id: I3c93f6bc4df553919dcbe05b00ef4c68f2c9ab60
Reviewed-on: https://webrtc-review.googlesource.com/87305
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23868}

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

@@ -16,7 +16,7 @@ import javax.annotation.Nullable;
  * Implements VideoCapturer.CapturerObserver and feeds frames to
  * webrtc::jni::AndroidVideoTrackSource.
  */
-class NativeCapturerObserver implements VideoCapturer.CapturerObserver {
+class NativeCapturerObserver implements CapturerObserver {
   // Pointer to webrtc::jni::AndroidVideoTrackSource.
   private final long nativeSource;
   // TODO(bugs.webrtc.org/9181): Remove.