Allow notification channels to be silent.

This better accomodates those apps that play their own sounds.
Since most apps don't play their own sounds, update the documentation
on notificationchannel to add guidance about when they should add a
sound.

Fixes: 37237013
Test: runtest systemui-notification
Change-Id: If00b15b1b44da66d24dacb2895e9a6c0a06d1890