In-call-notif doesn't use AUDIBILITY_ENFORCED flag

Audibilty enforced flag should only be used to make sure system
sounds are audible when this is mandated by a given country regulation.
Here, it was used incorrectly.  Also, adjusted in-call notification
sound to be less loud.

Fixes: 66808711
Test: manual

Change-Id: I119a718fb5fc605b6f6b3e387e5e9f956cfd5fff