Tag spam network traffic using TrafficStats.

The dialer app crashes on launch right now when app data is empty, because we fetch the spam list on app start, and we don't tag the network traffic using TrafficStats, which is a strict mode violation.

This CL tags spam network traffic using TrafficStats, by providing a custom transport executor for the GRPC channel. All activity on the channel is tagged as spam.

This unfortunately required creating a new thread pool. I am not sure if this creates more threads than before, because it is not clear what type of transport executor grpc uses by default. Note that the new thread pool will kill unused threads after 60 seconds so won't consume resources when idle.

Bug: 62388129
Test: manually verified app does not crash on launch with empty data. Can add automated tests upon request.
PiperOrigin-RevId: 161568882
Change-Id: I99da6ebb649fe1a63d634871ab314d8d910658f9