Have test URLs replace all validation URLs

Test URLs did not apply when an overlay set multiple validation URLs
through R.array.config_captive_portal_https_urls. This was not intended
as test URLs are designed to override all URL configuration so tests can
have predictable behavior.

Bug: 163119659
Test: atest NetworkMonitorTest
Original-Change: https://android-review.googlesource.com/1406760
Merged-In: Ic39a74b41a538afefed2ca08b43cfb7d324a5b78
Change-Id: Ic39a74b41a538afefed2ca08b43cfb7d324a5b78