CellularDataService: mCallbackMap is not thread safe

The HashMap object mCallbackMap in CellularDataService is a shared
resource between DataService's handler thread and CellularDataService's
handler thread. The update operations (put and remove) has
synchronization issue.

This CL fixes the issue by letting CellularDataService shares the
looper/handler thread of DataService (but keeping its own handler). And
thus all the operations of mCallbackMap are handled serializally in
DataService's handler thread.

One alternative solution is to  move all operations of the mCallbackMap
into
CellularDataService's handler thread to make it thread-safe. But it's
difficult for close() method which need to clear mCallbackMap and
shutdown the handler thread.

The other alternative solution is to replace HashMap with
ConcurrentHashMap
for mCallbackMap. This do make mCallbackMap thread-safe but do not
change the fact that it is still a shared resource between two threads.

Bug: 151103522
Test: atest FrameworksTelephonyTests
Change-Id: I7f5a507c2a79c7c77c5c2d7d3be147af80dc9f69