Annotate libjingle_peerconnection_java with @Nullable.

Bug: webrtc:8881
Change-Id: Ida2ef6c003567d19529c21629c916ed40e8de3a6
Reviewed-on: https://webrtc-review.googlesource.com/63380
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22563}

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

@@ -10,7 +10,10 @@
 
 package org.webrtc;
 
+import javax.annotation.Nullable;
+
 public class SoftwareVideoDecoderFactory implements VideoDecoderFactory {
+  @Nullable
   @Override
   public VideoDecoder createDecoder(String codecType) {
     if (codecType.equalsIgnoreCase("VP8")) {