Differentiate depleted and unset multipath quota

NetworkPolicyManagerInternalImpl#getSubscriptionOpportunisticQuota
would return 0 both for unset and depleted quota, which cannot be
interpreted properly by MultipathPolicyTracker (the only caller),
to use a fallback policy.

Return OPPORTUNISTIC_QUOTA_UNKNOWN (-1) instead for unknown quota.

Test: verified with CTS tests in ag/3811166
Bug: b/72631572
Bug: b/72877610
Change-Id: Ic087f7bfa9f10cc376b5ccd68a4685bd5242c586