fix back button issue

[okhiroyuki]

fix back button issue

when the user clicks the back button on the barcode scanning screen, the onKeyDown (in CaptureActivity) will react to it and the activity will be closed.
But the next Activity will capture the onBackPressed event, therefore it will be closed as well.
So with one back click, 2 Activity are closed.
phonegap/phonegap-plugin-barcodescanner#773