Having an object per interface will make it easier to support
properties.

Ideally, properties are reported in context in all responses. However
current implementation reports them only in response to a ReportState
request. This seems to work sufficiently. As long as the device is
opened in the Alexa app, Amazon will poll the device state every few
seconds with a ReportState request.

Fixes (mostly) home-assistant#11874.

Other interfaces will need properties implemented as well.

Implementing properties for just PowerController seems sufficient to
eliminate the "There was a problem." error for any device that supports
it, even if other interfaces are supported. Of course the additional
properties will be reported incorrectly in the Alexa app.

Includes a minor bugfix: `reportable` was previously placed incorrectly
in the responses, so Amazon was ignoring it.