Prepare some ConnectivityService logic for fallback to Cellular change

Reduce the duplication of some logic so when falling back to Cellular
when WiFi fails to validate is enabled, there's less chance for bugs
and failures:
1. De-duplicate several Network vs NetworkRequest matching functions
2. Remove the very tricky nascent logic by adding a simple "lingering" bit.

Bug:20896761
Change-Id: I21da9e827eec9cfd6835fcaa650192b9186ed053