Offer to cache ringtones in system DE storage.

Ringtones often live on shared media, which is now encrypted with CE
keys and not available until after the user is unlocked.  To improve
the user experience while locked, cache the default ringtone,
notification sound, and alarm sound in a DE storage area.

Bug: 26730753
Change-Id: Ie6ad7790af4c87dd25759df3ed017e3b91a2fb87