Make few of the new IMS API methods final

d8d03a8e added new methods to
InputMethodService.
Allowing IME developers to override some of these methods would introduce
unnecessary complexity. Making these methods final makes sure developers
cannot inject some unexpected behavior when called by system.

Bug: 73701052
Test: atest InputMethodServiceTest
Test: atest CtsInputMethodServiceHostTestCases
Change-Id: I1af4ed05a0b7306d840a21129e921a57b4a5fc33