Refactor async_turn_on() for ZHA Light. (#21156)

* Refactor async_turn_on() for ZHA Light.

Use "move_to_level_with_on_off" if brightness or transition attributes
are present in the service call data, otherwise issue "On" Zigbee
command.
Allow brightness of 0 for service call -- effectively turning the light
off.
Send color commands only after the light was turned on.

* Fix zha.light tests.