Describe the bug
This is hardcode from start just to have something set. Should calculate track length and use that instead.

To Reproduce

• Play something and with length not equal to three seconds
• Verify reported time - will always be three seconds (e.g. via control center)

Expected behavior
Length should reflect actual length of what is played.

System Setup (please complete the following information):

• OS: any
• Python: any
• pyatv: master
• Apple TV: any AirPlay receiver

Additional context
Just a minor thing I want to fix