Add Notification channels

In this iteration:
-Every app gets a default channel that notifications will be posted
to if they don't specify a channel themselves. The default channel
inherits app-wide settings on upgrade.
-Apps can create new channels without user approval, but apps
cannot change the name of a channel once created, nor can they ever
set the importance.
- When a notification is posted:
  - If the channel is marked as 'vibrates', vibration will be
    applied to notifications that lack a vibration. unlike the default
    notification flag, notifications will retain their custom vibration
    if given
  - Same with sound and lights
  - A notification's importance is the min of the app and channel
    importance
  - A notification can bypass dnd if: either the app or channel settings
    say it can
  - A notification's show on lockscreen setting comes from the app first,
    and the channel second if the app has no preference

Tests: in cl. also there's a cl for cts and a test app.

Change-Id: I630f99df655800b00586dcfab538d320d04fe0f0

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