Unininstall notifications -> notification channels

Otherwise the uninstall failed notifications are very hidden.

Bonus: IPackageInstall.uninstall thrown IAE when package is already
       uninstalled, at least do not crash.

Test: Faked a failure to uninstall
      Slowed down uninstall to see uninstall notification
Fixes: 35760276

Change-Id: I0ef09e4334df90ac8a70f28149ab4445bcdb60f4
(cherry picked from commit e5bb8d91)