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