NetworkListStoreData: Set creatorUid for all networks on load

Some network configurations saved during N have the creatorUid set to -1.
(Not sure what was the root cause in N which introduced this issue in
saved networks)
Ensure that we set |creatorUid| for such networks with invalid uid to
SYSTEM_UID on loading. The networks with invalid creatorUid can
anyway only be modified only by settings/sysui (because they possess
NETWORK_SETTINGS permission).

Bug: 65623732
Test: Unit tests
Change-Id: Ib6f61b81e19877cdf0c42d29c9f5ae3869725517
4 files changed