[bugfix] Stack Regression: synchronize m_itemCurrentFile with player state

[Voyager1]

Description

As reported on the team slack channel, when playing music hitting "next" lost current item information.
Upon deeper investigation, the culprit was with how m_itemCurrentFile is not properly synchronized with player state. This PR addresses that issue.
Additionally two special issues had to be dealt with:

• player doesn't always start gapless, in which case two player threads can be active (the one stopping plus the new one starting). In order to properly set m_itemCurrentFile, one must wait that the first player stopped before starting the new one.
• because m_itemCurrentFile is now reset to null once player is finished, there are a few additional checks on the validity of m_itemCurrentFile before dereferencing...

Motivation and Context

see above

How Has This Been Tested?

extensive mix of music, video, dvd, stacks, iso stacks.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[Voyager1]

closing in favor of #13301