Fix bluetooth settings pairing page stuck

There is heavy work to do when add device to list in the
DeviceListPreferenceFragment, off load these work from main thread
to solve the issue.

Make devicePreferenceMap a ConcurrentHashMap to avoid potential race
condition.

Also no longer use getCachedPreference(key) since we not put anything
into the cache, the fallback flow is always used.

Also in BluetoothDevicePreference.onPreferenceAttributesChanged(), move
more heavy work to background thread.

Using System.currentTimeMillis() to sort devices could cause flaky
because System.currentTimeMillis() could be same for different device,
use AtomicInteger instead.

Fix: 286628533
Test: Following the step in bug
Change-Id: Ia9750adb6b4c1424d084381e9d7c2ca8e7912391