Add accessibility floating menu settings key

* ACCESSIBILITY_FLOATING_MENU_SIZE: control different size
* ACCESSIBILITY_FLOATING_MENU_ICON_TYPE: control icon type
* ACCESSIBILITY_FLOATING_MENU_OPACITY: control the opacity value for fade out effect

Bug: 173940865
Test: atest SettingsProviderTest
Change-Id: I0399366b6ea05268e3d3ad1f24962fffde8d686a