[SP12.1] Add VtDataUsageProvider

This change contains minimum modifications to implement
AbstractNetworkStatsProvider which allows VT to report stats
asynchronously and remove the usage of getVtDataUsage.

Test: atest FrameworksNetTests FrameworksTelephonyTests
Bug: 130855321
Change-Id: I536352e8fb1b1f2a09e465db84916b0e6d463064
Fix: 143923500