Telephony: Set the default uri for Ringer according to different sim cards

When the phone ringtone is set as "None", getActualRingtoneUriBySubId()
in framework return null.But null won't be set to Ringer, so the uri
is still the value which was set in the last time.

Set the default uri in Settings.System to the Ringer instead of
the uri returned from the method before.

CRs-Fixed: 661216

Change-Id: I82dd48a8946eebdca477dfae0e09b4ed640e481c