Fix #3650 - Fix Playlist losing track of time when backgrounding or leaving the screen

[Brandon-T]

Summary of Changes

• Fix Playlist losing track of time when backgrounding the app or leaving the screen.
• Added observers for termination & backgrounding
• Added observer for item status to know when to seek to the correct playback time due to player race condition.

This pull request fixes #3650

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).