Add an API that allows VPNs to declare themselves as metered.

For VPN apps targeting Q and above, they will by default be treated as
metered unless they override this setting before establishing VPN.

Bug: 120145746
Test: atest FrameworksNetTests
Test: On device tests verifying meteredness setup correctly for apps
targeting Q and apps targeting P.
Change-Id: Ia6d1f7ef244bc04ae2e28faa59625302b5994875