Remove SCHEDULE_EXACT_ALARM from Shell

Shell doesn't need this permission and it confusing to the user
if visible in Settings.

Test: Shell doesn't appear in Settings under
Settings -> Apps -> Special app access -> Alarms & Reminders ->
3 dot menu -> Show system

Bug: 190775895
Change-Id: I98b051f37f3edf4f616f8847f691b956dafbdd12