Limit use of `cmd notification` to root and shell users only.

Only the shell and system can use notification manager shell
commands now. (Apps have APIs for that.)

Also add a few new notification shell commands:

  list -> lists keys of active notifications
  get <key> -> dumps the NotificationRecord
  snoozed -> lists snoozed notifications
  snooze (--for <ms> | --context <contextstring>) <key>
  unsnooze <key>

Bug: 150629389
Test: the shell commands themselves
Test: atest com.android.server.notification.NotificationShellCmdTest
Change-Id: Ie949821fcf1192efe8b6b3e95c9daca71347b11e