Add CaptivePortalApiUrl to DhcpResults

The URL will be used by DhcpClient to return it in its results.
It will not be parceled in DhcpResultsParcelable, but instead sent
through LinkProperties to network agents.

Bug: 139269711
Test: atest NetworkStackTests with associated NetworkStack change

Change-Id: I4ec9e7f5efece3ede9b0da5eb1b75d8d43b94ba9