Subscriber Id Match Rule won't be exposed as system Api.
Callers should use Builder#setSubscriberIds or
NetworkTemplate#getSubscriberIds to set/get the Subscriber
Ids filter. An empty set in the paremter or return value
represents the intention of matching any Subscriber Ids.
Test: atest NetworkPolicyManagerServiceTest \
FrameworksNetTests
Bug: 204830222
Change-Id: I12c54280b44c71e4737e82859dfe869794cc9c29