Use android Nullable instead of javax Nullable

This is a propagation of upstream chromium change needed to
resume DEPS autorolls into WebRTC.

Original comment from upstream change:

> This change is made in preparation for an ErrorProne
> check to catch this at compile time. See bug for details.

Bug: chromium:771683
Change-Id: I56aed15f73a633dcadae7ece6c645cd3596f9257
Reviewed-on: https://webrtc-review.googlesource.com/c/113505
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Artem Titarenko <artit@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25951}

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

@@ -10,10 +10,10 @@
 
 package org.webrtc;
 
+import android.support.annotation.Nullable;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
-import javax.annotation.Nullable;
 
 public class SoftwareVideoDecoderFactory implements VideoDecoderFactory {
   @Deprecated