add post_notifications permission to manifest

With the new Notifications Permission in Android T, packages
targeting SDK 31 need this permission declared in their manifest
in order to continue posting notifications.

Ignore-AOSP-First: Permission not yet publicly announced
Test: build, flash, and boot successfully
Bug: 194833441
Change-Id: Ia7d3c42c4af7b9638faa5665071650bc04e5b0de