Make sure InputMethodService's StylusWindowIdleTimeout always work

This changes InputMethodService#mHandler a non-null final.
This makes sure that #scheduleStylusWindowIdleTimeout() always works as
expected, independently to the service's state.

Without this change, stylus window idle timeout works only when
scheduleHandwritingSessionTimeout() or scheduleImeSurfaceRemoval() is
called before. Other than that, no behavior changes are expected.

Bug: 434580675
Test: StylusHandwritingTest#testHandwritingWindowRemoval_immediate
Flag: EXEMPT bug fix
Change-Id: I9ac1200aeab53b250a2791e93a092466bb67c765