Initial radio implementation

Initial radio implementation

Notable changes:
- Add radio gRPC API protobuf definitation and generated files.
- Reworked existing single gRPC APIs library to split it into
  per-API libraries to avoid name collision issues.
- Add radio gRPC client class and associated radio state class
  and RiverPod providers.
- Split media controls and play list table classes into media
  player and radio specific versions to facilitate customization
  and wiring up their appropriate backends in a straightforward
  fashion.  Some potential rationalization of styling widgets
  may be done as a follow up to avoid some duplication.
- Added radio configuration and presets loading.  The presets
  will be populated with the contents of a radio-presets.yaml
  file from the configured location, the default location is
  the /etc/xdg/AGL/ics-homescreen directory.
- Implemented FM radio player against the radio gRPC API.
  For the sake of expediency, no attempt has been made to make
  the player able to handle AM band support.
- Reworked media page navigation state so that active player is
  restored when coming back to the page.  Logic has been added to
  start/stop the radio on navigating to or leaving the FM radio
  sub-page.  This will potentially be reworked before CES to work
  with the pause/stop button present on the other pages.
- Started pruning down global exports.dart a bit to remove files
  only used in a specific page/hierarchy, starting with media.

Bug-AGL: SPEC-5029

Change-Id: I1ae0aca4a7a8218e69e4286c863f01509a1cccb7
Signed-off-by: Scott Murray <scott.murray@konsulko.com>