Set correct owner UID for VPN agentConnect()

This commit changes agentConnect to set the owner UID as the mOwnerUid
field instead of the Binder.getCallingUid().

Binder.getCallingUid() can return incorrect results for platform VPNs,
as agentConnect() is called under a clean calling UID.

Additionally, this relaxes the ownerUid sanitization check to allow a
VPN network's owner to see it's own ownership information.

Vpn.mOwnerUid is guaranteed to be correct, as all VPNs MUST have called
prepareInternal() at some previous point, which sets mOwnerUid as the
package's UID (or SYSTEM_UID if this is legacy VPN).

Bug: 150135470
Test: CTS tests showing ownership information
Change-Id: Ic979dad73983d722365849fbfb0becfd432b894c