parse_line: allow -1 for apexes and sdk libraries.

Since we're in much worse trouble if `/data/system/packages.list` is
attacker-controlled, there doesn't seem like much benefit to having
the little bit of [incomplete] range checking we had on the uid
field (by using a wider type than `uid_t` actually is), and
apparently we're now abusing `-1` to mean "apex or sdk library",
despite `uid_t` being an unsigned type.

Bug: http://b/336659478
Change-Id: I7a270eea937d21fc1d7fcda8654054210cf631fe