android: handle runtime permissions: - some mandatory permissions are requested on Cagou start, Cagou won't start at all and display a warning message if they are not granted (we request 5 times before showing the warning) - transfer plugin can now use "android_permissions" in plugin_info, to indicate what is necessary. The permissions will then be requested, and the plugin widget won't be shown if they are not granted (and a warning not will then be displayed)