Fix audio demuxer contiguity with segments not carrying ID3 timestamps

[robwalch]

This PR will...

1. Fix audio demuxer contiguity with segments not carrying ID3 timestamps
2. Clear audio stream controller waiting for init PTS data when loading state is reset
3. Fix progressive mode contiguity mux flag

Why is this Pull Request needed?

1. Audio segments missing ID3 timestamps were assigned incorrect start times and not appended correctly.
2. When seeking in streams with discontinuities while waiting for video to load in the previous discontinuity for the first time, processing of audio segments was not handled correctly.
3. When "progressive" mode was enabled, the transmuxer was reset on each chunk loaded leading to gaps in appended video.

Resolves issues:

Fixes #4835

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md