Using an External Usb Camera in Android

Could somebody give me an instruction how to use an external camera in android app. My app need to use front/back and external cameras but there is no official external camera API I found some issues about this, but they dont have an full answer how to realize this functionalitty especially in android API 21+ versions. As I understand, Camera2 API supports external camera and I can use this API for displaying preview from usb-camera, but most of devices can not see connected external camera. How can I fix this and how can I get this camera id when receive cameras list

String[] cameraList = manager.getCameraIdList();

About "devices can not see usb-camera" I find a few answers : USB camera can not be connected to Android in general, because Android4.0 USB host function has some problems about isochronous transfer..

http://www.infinitegra.co.jp/en/solution/AndroidUsbCam.htm

and maby camera does not getting power from usb-port

http://stackoverflow.com/a/8986688/5219832

But it written about old camera API. And I dont know, for example, how get power from usb-port.

Please give me an answer how correctly work with external camera