Introduce InputMethodMap#equals()

This CL enables us to compare two InputMethodMap instances to see
whether they contain exactly the same data or not. Such comparison
would be useful to skip unnecessary operations when there was no
update in the available IMEs.

This CL only adds utility methods, which will be used in subsequent
CLs. Thus there must be no observable behavior change.

Bug: 329703038
Test: atest FrameworksInputMethodSystemServerTests:InputMethodInfoUtilsTest
Test: atest FrameworksInputMethodSystemServerTests:InputMethodMapTest
Change-Id: I944f93a3fd45cfa6367ca396cf0cc45efd29c0e2