Add a supported type 'gesture' under ACCESSIBILITY_BUTTON_MODE

Root cause: From the user feedback, accessibiltiy gesture has more
benifits than floating accessibility button especially to low vision users.

Solution: Bring back accessibility gesture to be one of the options of
the accessibility button.

Bug: 190563948
Test: install new SettingsProvider.apk & adb reboot & manual test
Change-Id: I59952c744cddfbdc01b906d39b2681858f0bb2ea