Fix Android playback events after orientation change

[TheJim92]

Description

On Android, rotating the device while playing a VR video causes the position, duration and state EventChannels to stop emitting events.

The player itself continues playing correctly, but Flutter no longer receives playback updates, causing the progress bar and playback timer to freeze.

Cause

dispose(true) is called during orientation changes.

Although the player is intentionally kept alive when isRebuilding == true, the EventChannel sinks are still cleared, so subsequent playback events are discarded.

Fix

Only clear the EventChannel sinks when the player is actually destroyed (isRebuilding == false).

This preserves the existing event subscriptions during orientation changes while keeping the original behaviour for a real dispose.

Tested
Android device
VR video playback
Device rotation while playing
Progress bar continues updating correctly after rotation