[Clock] Alarm sounds page should show tick icon instead of checkbox to indicate chosen alarm sound

[Nekhelesh Ramananthan]

If you look at the clock alarm sound page (See attached screenshot), the checkbox used there is incorrect for several reasons as listed below,

1. It is a bit difficult to press something that small which is why we *also* allow users to press on the list item to choose that alarm sound.

2. Checkbox as explained by mtp here (https://bugs.launchpad.net/ubuntu/+source/ubuntu-ui-toolkit/+bug/1360840/comments/3) are supposed to be independent. We should either be using radio-buttons (which the SDK doesn''t provide) or just show tick-marks to indicate the current alarm sound is selected

3. We should maintain consistency with the ubuntu-system-settings app that uses tick-marks to indicate the current chosen ringtone. (See attached screenshot)

----- Design resolution -----
Replace checkboxes with tick-marks as used by the the ubuntu-settings-app.

[Ubuntu Phone Apps Jenkins Bot (ubuntu-phone-apps-jenkins-bot)]

Fix committed into lp:ubuntu-clock-app at revision 370, scheduled for release in ubuntu-clock-app, milestone 3.6