Improve code coverage for httplive_fuzzer

In order to improve coverage, updated the corpus files to include
both m3u8 and media files separated by a marker and updated the
fuzzer plugin to generate m3u8 and media files based on markers.

exec/s: 51
Test: ./httplive_fuzzer
Bug: 187131531

Change-Id: I262a79f2db37a0333c9a4e0fb07d426f34dda00e