Move "metered" persistence to WifiConfiguration.

For a long time we've had a nasty tangled dependency between Wi-Fi
and NPMS, since they both persisted different details for configured
networks.  As part of preparing for new carrier data plan APIs, move
the tracking of meteredness over to WifiConfiguration.

We've never really supported "unmetered" mobile networks inside the
framework, so remove the option to configure those flags.

Replace MeteredPreference.notifyChanged() with explicit listener
to avoid confusing an invalidation pass as expression of user intent
to change metered values.

Bug: 63391323
Test: builds, boots, Wi-Fi policy is upgraded
Exempt-From-Owner-Approval: No owner was found for changed files.
Change-Id: If8a6e12650d1d060a729ed1a80f07ebb65e0ffd7