Introduce InputMethodDialogWindowContext for testability

This is a follow up CL to our previous CLs [1], which introduced the
following classes for better testability.

 * com.android.server.inputmethod.InputMethodMenuController
 * com.android.server.wm.InputMethodMenuControllerTest

This CL only aims to clarify what we are currently testing.  Other
motivations of this CL are:

 * InputMethodMenuController is now a package-private final class,
   which is expected to be much easier to keep maintaining.
 * We can get rid of mock(InputMethodManagerService.class), which
   prevents us from making InputMethodManagerService a final class.

Anyway, this is a mechanical refactoring CL.  There should be no
observable behavior change.

 [1]: Ief3c691029b14d848f6ad4a5ec8a8743760c7cda
      c88452c3
 [2]: I8adb0c569ea6c208aee659603ee1d69df9939267
      9329fbdd

Bug: 192412909
Test: atest WmTests:InputMethodDialogWindowContextTest
Change-Id: I381b1e09823a7f8361a74d809d4b2fbd23eb556e