Refactor out TestHttpServer to share it between CaptivePortalApiTest and CaptivePortalTest, move it to frameworks/libs/net, and add a NetworkValidationTestUtil class with utilities to set the test validation URLs. Test: atest CtsNetTestCasesLatestSdk:CaptivePortal[Api]Test Bug: 160617623 Bug: 160656765 Change-Id: Icd7829e680b2dddd1ddaa3dc2d946c14c20b5a15