Give platform permissions a dummy group

We still don't want apps to deal with permissions groups but many apps
crash if the group is not set.

Test: android.permission2.cts.PermissionPolicyTest#testPlatformPermissionPolicyUnaltered
Change-Id: Ibc817d4d44a9a76a1c5fac170f8e76f96a9e4201
Fixes: 119836955