Fix security hole in GateKeeperResponse.

GateKeeperResponse has inconsistent writeToParcel() and
createFromParcel() methods, making it possible for a malicious app to
create a Bundle that changes contents after reserialization. Such
Bundles can be used to execute Intents with system privileges.

This CL changes writeToParcel() to make serialization and
deserialization consistent, thus fixing the issue.

Bug: 62998805
Test: use the debug app (see bug)
Change-Id: Ie1c64172c454c3a4b7a0919eb3454f0e38efcd09

core/java/android/service/gatekeeper/GateKeeperResponse.java

@@ -85,6 +85,8 @@ public final class GateKeeperResponse implements Parcelable {
             if (mPayload != null) {
                 dest.writeInt(mPayload.length);
                 dest.writeByteArray(mPayload);
+            } else {
+                dest.writeInt(0);
             }
         }
     }