Annotate lock-free method in InputMethodManagerInternal

This is a follow up CL to my previous CL [1], which attempted to
clarify what InputMethodManagerInternal method take lock internally.

To make code reading easier, this CL makes it clear about what method
internally acquire ImfLock and what method are always thread-safe in
terms of ImfLock.class.

Basically this is a mechanical code refactoring.  There should be no
observable behavior change.

 [1]: I98958e6c149582b0a24997c9ca814e355f8b2a2b
      0fee8d10

Bug: 343601565
Test: presubmit
Change-Id: I8b981d113d4b2664f830ccfaeaf5fe3a5fc5e61c