Bluetooth kept visible if page is not closed properly

[Michael Zanetti]

Steps to reproduce:

* Open the settings app
* enter the Bluetooth settings page
* close the settings app (do not exit the Bluetooth page)

=> device can still be found by other hosts and hciconfig confirms that ISCAN is still enabled.

As Bluetooth ISCAN mode consumes a significant amount of battery, ISCAN mode should be disabled when the app is closed or inactive.

From the bluez docs:

  boolean Discoverable [readwrite]

   Switch an adapter to discoverable or non-discoverable
   to either make it visible or hide it. This is a global
   setting and should only be used by the settings
   application.

   If the DiscoverableTimeout is set to a non-zero
   value then the system will set this value back to
   false after the timer expired.

So it would seem to me the best approach would be to pass a non-zero value for DiscoverableTimeout, something like 30 secs, so that the stack disables ISCAN mode on it's own if the ui isn't actively calling this again every 30 secs.