Fix: Initialize sonos timeline cache timestamp

[nesati]

Description

_timeline_cache_timestamp is uninitalised in PlexSonosClient. I added a line that initilises it the same way the base PlexClient does.

Fixes #1523

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ ] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated the docstring for new or existing methods
• [ ] I have added tests when applicable