Improve lock annotation for InputMethodManager (1/N)

In InputMethodManager, we used the "mH" as a lock to
protect some fields and methods for thread safe.

Some of them are already protected with this lock but
they didn't have the annotation yet.

Add the annotation for those fields to prevent unexpected
behavior change and increase the readability.

Bug: b/236937383
Test: presubmit
Change-Id: I3b345da25841b328358d16695e3a29ee65ae8d83