Add assist content admin control

Add permission to gate the admin control

This user restriction specifies if assist content is
disallowed from being sent to a privileged app such as
the Assistant app. Assist content includes screenshots
and information about an app, such as package name.

The admin control will be available to both device owners
and profile owners.

Bug: 322975406
Test: Compiles

Tests will be added to test the enforcement once that is implemented

Change-Id: I63688038fbb8b14d349da02c47237284381934e5