Mark APIs as 'final' as requested by Metalava.

Per Metalava guidance these should be marked final because we do
not expect developers who extend AccessibilityService to override
these APIs.

Fix: 340879733
Flag: EXEMPT refactor
Test: build & run accessibility CTS presubmit tests
Change-Id: I758c6983e8928f0e5918a53cceb5efa2aa12733e