fix several audio issues

[panentheos]

Summary of changes

This fixes three audio issues:

1. When partial early AM suppression on a mezzanine sign results in two headway messages, the resulting combined message ends up with both a destination and routes. The headway TTS audio code wasn't accounting for that possibility, resulting in a crash.
2. In the same case as above, the non-TTS audio code was using a slightly different algorithm than the text code, resulting in a headway readout using the top line's destination, instead of the route name.
3. The TTS audio code for station-level service ended messages was still saying "This station is closed" instead of using the line name.

[github-actions]

Coverage of commit e48a9ed

Summary coverage rate:
  lines......: 73.3% (2024 of 2761 lines)
  functions..: 75.4% (567 of 752 functions)
  branches...: no data found

Files changed coverage rate:
                                                             |Lines       |Functions  |Branches    
  Filename                                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================================
  lib/content/audio/service_ended.ex                         |86.4%     22|71.4%     7|    -      0
  lib/content/audio/vehicles_to_destination.ex               |36.6%     41| 100%     9|    -      0

Download coverage report

[cmaddox5]

Looks good to me.