Skip user set denied POST_NOTIFICATION requests for S- apps

For some reason, if multiple notification prompts are triggered, their
onCreate methods are queued, the second firing only after the first
activity finishes. To fix this, don't display a notification prompt for
pre-T apps if the permission is user set.

Fixes: 228319925
Test: manual
Change-Id: Ib0fd388de6bd372f3808feff3b779fa0c513cb3b