Add onBlockedStatusChanged(Network, int) to NetworkCallback.

This is similar to onBlockedStatusChanged(Network, boolean) but
it allows the callback holder to know the exact reason why
networking was blocked. It is useful to privileged system
components such as JobScheduler that are able to ignore some
blocked reasons but not others.

Also add a new BLOCKED_REASON_LOCKDOWN_VPN that is used when
networking is blocked because an always-on VPN is in
lockdown mode.

Also move BLOCKED_METERED_REASON_MASK to ConnectivityManager.
This is necessary because ConnectivityService must ensure that
the blocked status callbacks are correctly sent when meteredness
changes (e.g., a UID that is blocked on metered networks will
become unblocked on a network that becomes unmetered). In order
to do this it needs to know which reasons apply only on metered
networks.

Bug: 165835257
Test: unit tests in subsequent CLs in the stack
Change-Id: I647db4f5a01280be220288e73ffa85c15bec9370