[IT4.21] Update the label type of idle timer

The first parameter of adding idle timer is an unique identity
to communicate between ConnectivityService and netd. Netd will
notify the activity change using the identity, so it's fine to
replace the legacy type with transport type since we should
deprecate the usage of legacy type.

Bug: 170598012
Test: atest FrameworksNetTests
Change-Id: Ia00606539b86872cca9a92285bd940c8a720a033