Use a concurrent hash map to access VolumeDialogControllerImpl's callbacks

The issue is that the VolumeDialogControllerImpl.C's mCallbackMap is
accessed from two different threads. Instrumentation shows that via
add() and remove() it is modified on the main thread, and methods like
C.onConfigurationChanged() are called from
VolumeDialogControllerImpl.Receiver, which registers with
on a handler tied to a background thread
(VolumeDialogControllerImpl.W/mWorker). C.onConfigurationChanged()
is called directly from that thread.

Use a concurrent hash map in place of hash map to avoid the
concurrent modification exception.

Test: manual. Ensure device boots. Ensure basic volume control
functions. Ensure a configuration change (via locale change)
does not result in any crash.

Bug: https://issuetracker.google.com/issues/150402778

Change-Id: I26bb8533c30d57e48f500e6863d0f522e1559567