Secondary media buttons are less frequently used controls of the playback mode and custom source-specific controls.
The list of controls is controlled by MediaButtonsConfiguration and can be updated when the MediaControlContext changes.
The maximum number of buttons is limited by MediaButtonsConfiguration.secondaryMediaButtonsLimit. If the number of controls is greater than the limit, controls with lower MediaControlPriority are filtered out. The order is preserved. If two controls have the same priority, the first in order is selected.