Inline InputMethodMenuController.HardKeyboardListener

This is a follow up CL to our previous CL [1], which introduced

   InputMethodManagerService.HardKeyboardListener,

which was later moved to

   InputMethodMenuController.HardKeyboardListener

as part of refactoring [2].

This CL mechanically inlines it to InputMethodManagerService as it
allows us to achieve the same behavior with fewer lines of code.

This is mechanical refactoring. There should be no observable behavior
change.

 [1]: Ica768083f95c33dc1e494a28ba7d8b6eb989b0ef
      01038492
 [2]: Ief3c691029b14d848f6ad4a5ec8a8743760c7cda
      c88452c3

Bug: 192412909
Test: Manually verified by attaching/detaching a hardware keyboard.
Change-Id: I9cce75011a8564c9da8307c5555d87bcba4db2cb