Add missing @GuardedBy to IMMS methods

This is a mechanical code clean-up that only adds missing

  @GuardedBy("mMethodMap")

annotations to methods in InputMethodManagerService for better static
code analysis.  There should be no observable behavior change.

Bug: 192412909
Test: presubmit
Change-Id: I88aa22af0e4512466bf7d13821243e72a32290ea