Make InputMethodManagerService a final class

This CL makes it clear that InputMethodManagerService should never be
extended nor mocked, and compilers and ART should be able to optimize
the code based on such an assumption.

There should be no observable behavior change in the production code.

Bug: 192412909
Test: presubmit
Change-Id: I203554fa2964eac9879a5ba5e23ba2f7087403ed