UVCプラグインのonDestroy時の挙動修正

## 修正内容
* UVCプラグインのonDestroy時にヌルチェックを追加。

Author: TakayukiHoshi1984

dConnectDevicePlugin/dConnectDeviceUVC/app/src/main/java/org/deviceconnect/android/deviceplugin/uvc/UVCDeviceService.java

@@ -78,9 +78,11 @@ public void onDestroy() {
                 ((UVCService) service).closeUVCDevice();
             }
         }
-        mDeviceMgr.removeDeviceListener(mDeviceListener);
-        mDeviceMgr.removeConnectionListener(mConnectionListener);
-        mDeviceMgr.stop();
+        if (mDeviceMgr != null) {
+            mDeviceMgr.removeDeviceListener(mDeviceListener);
+            mDeviceMgr.removeConnectionListener(mConnectionListener);
+            mDeviceMgr.stop();
+        }
         unregisterReceiver(mPermissionReceiver);
         super.onDestroy();
     }