[3/3] Settings: Audio Themes Support

from: https://gerrit.omnirom.org/#/c/5245/

PS 2&3: Update author
PS4: Rebased & fix spaces
Ps5: Move the preference under System sound header
Conflicts:

res/values/pac_strings.xml

change-Id: I6a92e4c9173be6f6ce31e8555902ea36e05bb82f