Add a property for the declarition of accessibility purpose

AccessibilityService is a powerful service that could view the content
or control the device. However, some apps using it is not for
accessibility. We would like to add "isAccessibilityTool" property in
the xml to distinguish its purpose.
If the Accessibilityservices is not for users with disabilities,
system would inform the user after enabling for a period of time.

Bug: 176960226
Test: atest CtsAccessibilityTestCases#AccessibilityServiceInfoTest
Change-Id: I91327470ce39a28e701fa6841f9edccc1a92ffe4