fix(android): send route_id and start_date; never report the route as trip_id

[aaronbrethorst]

Summary

Second PR in the trip-descriptor stack (stacked on the server PR). The Android driver app used to copy the route id into trip_id whenever the driver left the GTFS trip id blank, so the feed published a route id as a trip id. Now:

• ActiveTrip stores gtfsTripId (blank when the driver only knows the route — never a route id) and startDate (service date, YYYYMMDD, the device-local calendar date when the trip started, fixed for the life of the trip).
• Every location report sends route_id and start_date; trip_id is sent only when a GTFS trip id was entered, and is omitted from the JSON entirely otherwise (explicitNulls = false).
• Both the entered route id and GTFS trip id are trimmed before they reach the trip-start request, local state or the wire, so "5 " can't publish an unmatchable route_id.
• A trip persisted by an older build (no new DataStore keys) still loads: blank GTFS id, start date derived from the stored start time; the legacy trip_location_id key is cleaned up on clear.
• ZoneId is provided by Hilt so the repository is testable with a fixed zone; tests cover the date-line rollover (23:30 UTC → next day in Nairobi).
• The smoke-test doc checks, after the first GPS fix, that the feed shows routeId/startDate and no tripId for a route-only trip, and adds a final check with a GTFS trip id filled in; the Android design spec's "server-side note" is marked resolved.

Requires the server from the previous PR: an older server rejects route_id/start_date as unknown fields with 400.

Test plan

• ./gradlew :app:testDebugUnitTest — new/updated: RepositoriesTest (gtfs id + route + Nairobi service date; blank gtfs id stays blank; serviceDate formatting), TrackerApiTest (exact key sets with and without trip_id), TripReporterTest (wire body with and without a GTFS id)
• ./gradlew :app:compileDebugAndroidTestKotlin
• Manual: follow docs/android-smoke-test.md (Check 2 and the final check) against a server running the previous PR

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