Revert "Adding Notification Channel"

* This is apparently causing issues for many users,
  while using plain old notifications doesn't.
  Setting API to 27 is also not helping, so just
  roll back to what AOSP shipped.

This reverts commit 3d59c126.

Change-Id: I4b6914f7fd1a1afc13cc38241ae8e5677e6620ea