Fix a bug where a NetworkAgent gets repeatedly torn down

mUids is not marshalled correctly when null so if the
NetworkAgent runs in another process and is not a VPN then
the system will see its allowed Uids as being the empty
list (= nobody can use this network) instead of a null
list (= everybody can use this network). This breaks
emulator networking.

Bug: 72436966
Test: runtests frameworks-net
Test: also manual testing, this does fix emulator networking
      and seems not to break phone networking
Change-Id: Id2bbf3808e80b19cd055c832c11cf72372710942