Clean up for multiple validation result update

This is a follow-up commit for aosp/955431 to update commets
and minor updates in unit test.

Test: atest com.android.server.ConnectivityServiceTest#testCaptivePortalOnPartialConnectivity
Bug: 130683832
Change-Id: I581eae8daeddd2c4c186e7b40e27fef2aaa7ab43