[Suggestion] Performance issue

[cybercamphackathon]

In java the String are immutable. So, do that is very inefficient:

String text;
text += line + "\r\n ";

The code do that in many places of the project, for example: https://github.com/borjalor/DroidSentinel/blob/master/DroidSentinel/app/src/main/java/com/example/droidsentinel/droidsentinel/LogTask.java#L237