Remove unused variables in file media_renderer.cc

Variables "objects" and "child" of type GVariant* were declared but never used.
That's why they were removed. Some other nits were fixed.
Merge pull request #418 from piotrowskim/rm_unused_media_renderer

Remove unused variables in file media_renderer.cc
[bluetooth] keep BT state when calling getDefaultAdapter() API

Bluetooth-frwk has been adapted to allow getting adapter properties
even if bluetooth adapter is powered off.

So modify getDefaultAdapter() API logic in order to be synchronous.

Handle bonded devices in order to fill known_devices array in javascript
once bluetooth is enabled.

relates to TC-2024

Signed-off-by: Corentin Lecouvey <corentin.lecouvey@open.eurogiciel.org>