Use the binder to identify keepalive in IConnectivityManager

This is much simpler and less error-prone, as well as less
subject to race conditions.

It also allows for cleaning up some TODOs.

Test: FrameworksNetTests
      CtsNetTestCases
Bug: 267116236
Change-Id: I470c709446946ef35a0324427defe2f58b434339
10 files changed