[pm] a system config to let oems register uids

BUG: 287574607
Test: manually enable the flag and set a test uid via the xml and verify
that the uid is added by adding logs. Notice that the newly added uid
will be cleared if there's no package installed with the specified shared uid.

(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:616d913df8e3bb896173b807b005efca3b82201d)
Change-Id: Ic436003390c8feb4c5e9a36106abc1f7ab7f5a46