Bluetooth: Always scan while on pairing or DevicePicker page

* Modified DeviceListPreferenceFragment to have enable/disable scanning
  methods. In ENABLE state, each onScanningStateChanged(false) will
  restart another round of scanning
* Subclasses of DeviceListPreferenceFragment should call enable/disable
  scanning when scanning is needed for long period of time
* Currently, BluetoothPairingDetail and DevicePickerFragment call
  enableScanning() when Bluetooth is turned ON and call disableScanning
  when some device is picked in their lists
* Both BluetoothPairingDetail and DevicePickerFragment will re-enable
  scanning if pairing failed for selected device
* Added associated unit tests as well

Bug: 32172815
Test: make, pair Bluetooth device, send file over Bluetooth Opp
Change-Id: I99325e06aadd7b00e7a7ba6d6c282a6831859d8b

src/com/android/settings/bluetooth/DeviceListPreferenceFragment.java

@@ -54,6 +54,9 @@ public abstract class DeviceListPreferenceFragment extends
 
     private BluetoothDeviceFilter.Filter mFilter;
 
+    @VisibleForTesting
+    boolean mScanEnabled;
+
     BluetoothDevice mSelectedDevice;
 
     LocalBluetoothAdapter mLocalAdapter;
@@ -216,8 +219,25 @@ public abstract class DeviceListPreferenceFragment extends
         }
     }
 
+    @VisibleForTesting
+    void enableScanning() {
+        // LocalBluetoothAdapter already handles repeated scan requests
+        mLocalAdapter.startScanning(true);
+        mScanEnabled = true;
+    }
+
+    @VisibleForTesting
+    void disableScanning() {
+        mLocalAdapter.stopScanning();
+        mScanEnabled = false;
+    }
+
     @Override
-    public void onScanningStateChanged(boolean started) {}
+    public void onScanningStateChanged(boolean started) {
+        if (!started && mScanEnabled) {
+            mLocalAdapter.startScanning(true);
+        }
+    }
 
     @Override
     public void onBluetoothStateChanged(int bluetoothState) {}