media:Modify the display of system default ringtone when it is none.

After we set the sound of calender use the default ringtone , if the
system default ringtone is none, the sound of calender will show
unknown ringtone.

Make sound of calender not show unknown ringtone when the system
default ringtone is none.

CRs-fixed: 812323
Change-Id: Idbc488748a81b88bb7a0df49e25d6a8cf6ba3826