Scale PS upload back to known committers

The current permissions allow anyone on the internet to driveby and
upload a PS.  Only active contributors to the project should be able
to do this sort of thing, so scale back access.

NB: Change owners can always upload new patchsets for their own CLs.

Bug: 507976856
Change-Id: I348f5e0614bfd03adf074ef7a9836244c4942242