Add a fallback lock file path (r/w) for Android

Not sure this is the most suboptimal solution, but since the QLockFile usage is
not optional in QtSerialPort as of now, it allows us to get going for the
release with Android.

Change-Id: I4ee2658f76f14061d2f7e69e218a152a7235e524
Reviewed-by: BogDan Vatra <bogdan@kde.org>