Fix dup preference key on Connected devices page

Use different key (prefix + CacheBluetooothDevice hash) for
each BluetoothDevicePreference to avoid Java crash due to "Two different
ViewHolders have the same change ID".

Test: atest
Fix: 325831741
Flag: EXEMPT safe fix only change preference key string
Change-Id: Ic993f3cab206b794045c2f46e6b4b376bfa3adea

tests/robotests/src/com/android/settings/bluetooth/BluetoothDeviceUpdaterTest.java

@@ -275,7 +275,7 @@ public class BluetoothDeviceUpdaterTest {
         }
 
         @Override
-        protected String getPreferenceKey() {
+        protected String getPreferenceKeyPrefix() {
             return "test_bt";
         }
     }