Add logging backend for Android

On some devices the Android platform will send messages send to the classical Unix syslog service to Android's ringbuffer log that can be read via a USB cable with the adb tool.

Since we are unsure if every Android device does this automatic conversion from Syslog to Android's logging system, it might make sense to have an "Log android" line for torrc like we currently have with "Log syslog" in the case we have a technical user with a device that does NOT send syslogs to the Android log who found, or is willing to debug, some issues for us.