Only use uids and gids for which we've allocated AIDs

Currently, getpwnam, getpwent, etc return successfully for any uid
from AID_APP_START (10000) to AID_USER_OFFSET (100000) for each user.
This is not correct however, as only specific ranges above
AID_APP_START are reserved as valid ranges.  This change corrects this.

This is particularly important as the newly added AID_OVERFLOWUID is
65534, which is above AID_APP_START but not in any reserved range,
collided with the faulty returned values.

Bug: 69119022
Bug: 69128408
Test: pwd/grp bionic unit tests
Change-Id: I3dae97a90597915fa30a88fe27cda88b107e9c35