android: Return -EUSERS when failed to open a Camera that's in use

The correct return value for the HAL for hal_dev_open() when trying to
open a camera that's already opened is EUSERS. Make hal_dev_open()
return -EUSERS, and plumb the logic for this through
CameraHalManager::open().

This allows the following CTS tests to pass:
- android.hardware.camera2.cts.CameraManagerTest#testCameraManagerOpenAllCameras
- android.hardware.camera2.cts.MultiViewTest#testDualCameraPreview

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

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