Simplify SoftInputWindow

Now that SoftInputWindow is used only by InputMethodService [1], we
can safely simplify SoftInputWindow.

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

 [1]: 5ed8dae6bc9cca87d5f35e781f173477a2388c2e

Bug: 192412909
Test: prebuilts/checkstyle/checkstyle.py -f \
  frameworks/base/core/java/android/inputmethodservice/SoftInputWindow.java
Change-Id: I619ae6011f5e66dba126931719569a97513eaa7e