Contributing all buttons in one place goes against the designs that we
set out at the beginning of the project's rewrite and that multiple of
us have been following since then.

Create standalone components for each feature and move all state to them.
Toolbars are now dummy containers.

Rearrange the ToolbarButtons in the secondary Toolbar in order to mostly
group the media-related ones such as the AudioRouteButton, the
switchCamera button, and the audio-only mode button.

Due to the difference in nature, the iOS and Android implementations are
completely different:

iOS: MPVolumeView is used, which allows us to place a button which will launch a
native route picker provided by iOS itself. This view is different depending on
the iOS version, with the iOS 11 version being more complete.

Android: A completely custom component is used, which displays a bottom sheet
with the device categories, not devices individually. This is akin to the sheet
in the builtin dialer.