Switch display uid whitelist from task to activity

Tasks can contain activities from multiple UIDs, so the uid whitelists
for private displays need to be based on activities instead of tasks.

This change also stops keeping track of uids on public displays, since
the uid list is not used for those displays.

Test: android.server.cts.ActivityManagerDisplayTests
Test: #testPermissionLaunchMultiUidTask
Change-Id: I38f1434f581af64f0db40221431994fd5a99bab3