Fix issue 3435692

The problem is that the AudioService is not notified of the death of
IAudioFlinger binder interface because no methods was called on AudioFlinger
from system_server since the device booted. Therefore, AudioSystem did not created any
IAudioFlinger interface in the system_server process.
The root cause was always present but more likely to happen on a tablet than on a phone
as for instance no beeps are played when volume is adjusted.

The fix consists in having AudioSystem explicitly call a method on AudioFlinger when a
callback is installed to make sure that an IAudioFlinger binder interface is created
regardless of any other activity in the client process.

Change-Id: I9df34a36825af0b25cd0246dd02edbd712263f41