Track multiple network activities on V+

on V+ devices, NetworkActivityTracker adds idleTimer when the network is
first connected and removes idleTimer when the network is disconnected
so that activity tracker can receive netd
onInterfaceClassActivityChanged callback for multiple networks.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I6f3a95c27e3e58f3f60c40065f562d00431a56b1