[Thread] fix upstream network selector to exclude VPN

Previously the upstream network selector wrongly cleared the default
network capabilities so there's a risk of choosing VPN as the upstream
network, which is undesired.

Bug: 356346764
Test: atest ThreadNetworkIntegrationTests
ThreadNetworkControllerServiceTest

Change-Id: Ic1762775734d734f1974a753f100814b61d66589
2 files changed