Bug 923647: Replace GetDefaultAdapterAdapter by non-blocking implementation, r=echou

GetDefaultAdapter blocks while querying the default Bluetooth
adapter. This patch replaces the function with a non-blocking
call to DBus. The reply is handled in a callback function.

As a side effect, this change also removes an invalid warning in
cases where no default adapter is available, and enabling Bluetooth
seems a bit faster.

--HG--
extra : rebase_source : ad3cbb2ccc8ac98b3c8e6d56e47822da0afb07f1