Notification Channels come to SystemUI's own notifications.

There are five channels at present:
    ALERTS - low battery etc.
    SCREENSHOTS - progress & result
    SECURITY - notifications hidden due to policy
    STORAGE - disk low
    STATUS - basically everything else

The importance for each channel should match the legacy
priority of the notifications it carries.

Bug: 34250937
Test: runtest systemui
Change-Id: I5915ca453258caea63b0d9bd756893db05e8d600