Register 'firetv.adb_command' service (#21419)

* Register 'media_player.firetv_adb_cmd' service

* Wrap the 'firetv_adb_cmd' service with 'adb_decorator'

* Address reviewer comments

* Move firetv to its own platform

* Move 'adb_command' service description

* Rename DOMAIN to FIRETV_DOMAIN

* Import KEYS in __init__ method

* Change 'self.KEYS' to 'self.keys'

* Update firetv in .coveragerc

* 'homeassistant.components.media_player.firetv' -> 'homeassistant.components.firetv'

* 'homeassistant.components.firetv' -> 'homeassistant.components.firetv.media_player'