Fix unresponsive UI during scan and display of "No Bluetooth devices"

- When there are no available Bluetooth devices, we show
  "No Bluetooth devices were found nearby" as a preference item after
  the scan completes. Update the layout for this Preference item so
  that the text is centered and doesn't highlight when touched.

- Fix slow responsiveness on scanning caused by repeated removal of
  "No Bluetooth devices were found" preference item on every call to
  onBindView().

- If no devices are paired, don't restart the scan on every rotation.

Bug: 5173935
Bug: 5055460
Change-Id: I52fb2d52f315c5e793f08499146842d54c5c3ead