Allow null IVoldMountCallback in Vold#mount

Test: m
Bug: 145231499
Change-Id: I51cb4424d821078a3604d5b3a42e99d3aaf658c3

binder/android/os/IVold.aidl

@@ -42,7 +42,7 @@ interface IVold {
     void forgetPartition(@utf8InCpp String partGuid, @utf8InCpp String fsUuid);
 
     void mount(@utf8InCpp String volId, int mountFlags, int mountUserId,
-         IVoldMountCallback callback);
+         @nullable IVoldMountCallback callback);
     void unmount(@utf8InCpp String volId);
     void format(@utf8InCpp String volId, @utf8InCpp String fsType);
     void benchmark(@utf8InCpp String volId, IVoldTaskListener listener);