Post instant app notifications to an instant apps channel

'General' was too undescriptive. Since the channel's importance
is fixed, and these notifications already break through DND,
I don't think it's worth migrating user settings from the old channel
to the new channel.

Test: NotificationChannelsTest
Test: run an instant app, verify channel
Fixes: 225171990
Change-Id: I79b8fe015db1bd177b998752ddee8c037dbf3e1b