Fix race condition when setting default ringtones

If the device was powered off during first boot, after media scanner
inserted some entries but before the default ringtone settings were
set (or committed to disk), the default settings would not be set
on subsequent boots.

Bug: 18625739
Bug: 22349910
Change-Id: Iff07da59a9c6d53bf2950bd107ee74d02b7f48d6