Move InputMethodManagerService to com.android.server.inputmethod

This CL moves InputMethodManagerService (IMMS)
  from com.android.server
    to com.android.server.inputmethod
so that we can mechanically factor out inner classes from IMMS to
separate package private classes.

This is purely a mechanical refactoring of an implementation detail.
There should be no observable behavior difference.

Fix: 114660660
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:com.android.internal.inputmethod
Change-Id: I023d8f2863601ee54f304988117d8ea750770f70