Don't backup ringtone on non-telephony devices.

Restoring a device that supports telephony using the backup set of
a non-telephony device would cause the ringtone to be set to "None"
instead of the default value. This was due to the fact that on
non-telephony devices, the ringtone value was being backed up as
"_silent" instead of null.

Bug: 18777629

Change-Id: Idece1f874438a895169dbba7df1d716adea6660e