C2 VTS: Update SetUp() to reduce code duplications

- Renamed and moved getting file name functions to SetUp
- Added checks to mark tests as skipped, when no valid files
  are found for a mime
- Removed references to URL as these were just file names
- Updated audio encoder tests to call getting supported
  sample rate and channel counts in SetUp

Bug: 183454262
Bug: 189154961

Test: atest VtsHalMediaC2V1_0TargetAudioDecTest \
 VtsHalMediaC2V1_0TargetAudioEncTest \
 VtsHalMediaC2V1_0TargetVideoDecTest \
 VtsHalMediaC2V1_0TargetVideoEncTest

Change-Id: Icf3cff3690140e3297299d3bb2ce7b9f24dfd791