Enable notifications during an ongoing call

If a phone call is ongoing, the alert will be allowed but
only using a special incall notification tone and no
vibration.  The tone will be played on the voice call audio
stream.

Also enable customization of the relative volume of the in
call notification volume. This allows the level to be
changed for different operators using overlays.

The requirement behind this is to allow audible SMS/MMS
notifications during phone calls, something that is
requested by a number of European operators.

Bug: 28688969
Change-Id: Ia8ce5c784a847e6759b55e6a78ac2100db47df19
Test: manual