ringtone: check the selected file

When a user selects a new ringtone, the application checks if:
- the format is not mp3 or mp2 because libsndfile does not support them
- the size is less than 800kB. If the size is too big, new incoming calls
fail because the ringtone is not buffered.

Change-Id: I96d487c6f9cdb145ef18d5fcb593642ba4b2acc1
Tuleap: #1117