Decouple DisplayImeController and IInputMethodManager

This CL removes the direct dependency on IInputMethodManager from
DisplayImeController by using IInputMethodManagerGlobal, which was
introduced recently [1].

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

 [1]: I5412eb1d44e3d515ca955f00a2e777b659a15b14

Fix: 192610976
Test: presubmit
Test: atest DisplayImeControllerTest
Change-Id: I9de401ad0a1051790d95fcfc8d2552a656281273