Update mActiveIdleTimers to use netId as a key

Following CL update +U devices to use netId as an idle timer label.
Using netId as key for mActiveIdleTimers makes it easier to handle
network activity change event in U+ devices.

Bug: 267870186
Bug: 279380356
Test: atest FrameworksNetTests
Change-Id: I4cba6fa546ab403b9a462cc7b77393ac0227f690
1 file changed