Use netId as idleTimer label on V+

Following CL adds idleTimer for multiple networks and track activities
including non default network on V+
Current LegacyNetworkActivityTracker uses transport type as an idleTimer
label but this makes it difficult to have multiple idleTimers.
So, this CL updates LegacyNetworkActivityTracker to support using netId
as an idleTimer label.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I0039f5624ae1d142dbacba53aa90ca9bf6d43599