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}
diff --git a/sdk/android/api/org/webrtc/JavaI420Buffer.java b/sdk/android/api/org/webrtc/JavaI420Buffer.java
index dce6a4c..0deb38f 100644
--- a/sdk/android/api/org/webrtc/JavaI420Buffer.java
+++ b/sdk/android/api/org/webrtc/JavaI420Buffer.java
@@ -10,8 +10,8 @@
 
 package org.webrtc;
 
+import android.support.annotation.Nullable;
 import java.nio.ByteBuffer;
-import javax.annotation.Nullable;
 import org.webrtc.VideoFrame.I420Buffer;
 
 /** Implementation of VideoFrame.I420Buffer backed by Java direct byte buffers. */