fix aiohttp InvalidURL exception when fetching media player image (#15572)

* fix aiohttp InvalidURL exception when fetching media player image

The first call for the HA proxy (`/api/media_player_proxy/media_player.kodi?token=...&cache=...`)
is receiving relative urls that are failing, this is a simple fix to precede the base_url when hostname is None.

* fix import location and sort stdlib imports