When a session sets a custom layout, we currently convert it to the
media button preferences in MediaControllerImplBase and convert it back
to a custom layout for consumers of MediaController.getCustomLayout.
To avoid unnecessary conversions and potential changes, we can
directly use the provided custom layout to get the same logic as
before. This also means we avoid notifying a change to the custom layout
if only the implicit slots changed that weren't relevant before
introducing them (=we can remove some tests for this behavior).
PiperOrigin-RevId: 701903475
da4376d48d