MediaTranscoding: Use MediaPreparer to download test files

Also,
- Updated the error logs to include file path.
- added a header file to define path to the media files.

Bug: 298733419
Test: atest MediaTranscoderBenchmark MediaSampleReaderBenchmark \
 MediaTrackTranscoderBenchmark

Change-Id: I037930a6a4c4e994497ba369483c8371172c376f