Add flag to separate notification audio and ui

AudioService and SeekBarVolumizer now read a flag instead of a config
boolean, when trying to determine if notification stream is separate
from ring.

Remove the now unused config & symbol for ring/notif stream alias

Bug: b/38477228

Test: atest AudioServiceTest#testRingNotifAlias

Change-Id: I1ee08c78984d3841711f88fcec360c936da00090