Don't enable or disable the Bluetooth adapter automatically

This branch no longer enables/disables the Bluetooth adapter when the user enables/disables the plugin. Likewise the adapter isn't enabled at startup if the plugin is enabled, or disabled at shutdown if we enabled it during the session.

This is likely to please some users and annoy others. It makes the behaviour of the Bluetooth toggle consistent with the wifi toggle, which was an issue raised in usability testing (see #1748).

The adapter is still enabled by the OS when adding a contact in person, if the user accepts the Bluetooth discoverability prompt.

Closes #1348 (closed)