Cannot turn off casper-a11y-enable settings

Ubiquity greeter mode call casper-a11y-enable from a11y_indicator_menu.
However when once call casper-a11y-enable, user cannot disable a11y
settings.

Needs to implement to reset settings option or to set toggle like.

Procedure:
1. Bootup from Ubuntu Desktop CD
2. Do nothing in gfxboot (to boot ubiquity with greeter option)
3. Displayed daialog to ask "Try Ubuntu" or "Install"
4. Displayed a11y indicator(blue circle) in top-right
5. Select "High Contrast" in a11y indicator
6. Exec casper-a11y-enable and turn on High Contrast mode
7. Select "High Contrast" in a11y indicator (unvisible indicator LP: #997064)

Expected result:
- turn off High Contrast mode (as like lightdm)

Actual result:
- nothing happen (stay on High Cntrast mode)