android: add rollback RTCSdpType

BUG=webrtc:11796,webrtc:11970

Change-Id: I0047c7a050c344ef58735d9d0d6534b1ddf6c4d6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/184263
Reviewed-by: Taylor <deadbeef@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#32243}

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

@@ -22,7 +22,8 @@ public class SessionDescription {
   public static enum Type {
     OFFER,
     PRANSWER,
-    ANSWER;
+    ANSWER,
+    ROLLBACK;
 
     public String canonicalForm() {
       return name().toLowerCase(Locale.US);