Added missing permission declaration on <service> tag of AccessibilityService guide documentation (Building Accessibility Services)

With code snippet on current docs, the accessibility service does not show up on the Settings > Accessibility > Services list, with API level 17 or higher. In order to declare service correctly, user have to declare "permission" property with value of "android.permission.BIND_ACCESSIBILITY_SERVICE".

Change-Id: I70c8f98d4ff6fd2286bf5866098b22443afb6cd9
Merge "Added missing permission declaration on <service> tag of AccessibilityService guide documentation (Building Accessibility Services)"