Bug 964817: Invoke DBus send operations on I/O thread, r=echou

Currently, the DBus connection for Bluetooth is shared between the
main thread and the I/O thread. This causes race conditions when
starting or stopping Bluetooth.

This patch moves all occurences of main-thread-invoked DBus send
operations to the I/O thread. This is mostly refactoring. Internally,
send operations are already executed on the main thread, so there is
no change in the over all logic of these methods.