[2/2] Power connect/disconnect notification support

part 1/2: frameworks/base PowerUI and Settings
http://review.cyanogenmod.org/#/c/35241/

part 2/2: packages/apps/Settings Sound settings
http://review.cyanogenmod.org/#/c/35242/

patchset 3:
    set settings persistence and dependencies in xml resource file
    shrink long lines
    add the default notification tone to the ringtone picker and
        remove the now redundant "use default" checkbox
    move values/strings.xml content to the cyanogenmod section
    improve (mostly delete) wording in values/strings.xml
    remove tab whitespace

Change-Id: I7ddd8a47ae4f9a62c586023d151ac42bbe8424c7