Add @Nullable annotations to quiet errorprone.

Those are preventive annotations to prepare for incoming android update
(coming with Chromium roll).
Currently the roll is blocked partly because errorprone complains!

Bug: webrtc:11095, chromium:1003532
Change-Id: If4e2879a522e895ce7fb1f2a9ad36d06f98f2a61
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160002
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@google.com>
Cr-Commit-Position: refs/heads/master@{#29830}

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

@@ -395,6 +395,7 @@ public class PeerConnection {
       }
     }
 
+    @Nullable
     @CalledByNative("AdapterType")
     static AdapterType fromNativeIndex(int nativeIndex) {
       return BY_BITMASK.get(nativeIndex);