This ensures that media button preferences with slots for BACK,
FORWARD and OVERFLOW are converted to the legacy custom layout
according to the implicit placement rules. This has to happen
when populating the MediaSessionCompat and when generating the
notification for older APIs. Sending these preferences to older
Media3 controllers can filter them down to the custom layout the
session would have used previously, but there is no need to adjust
the reservation extras because the older Media3 custom layout has
no concept of these extras.
PiperOrigin-RevId: 689761637
a15571c8ee