Fix lint errors and style nits in SoftInputWindow

This is a mechanical refactoring CL as a preparation to remove the
dependency on

   android.inputmethodservice.SoftInputWindow

from

   android.service.voice.VoiceInteractionSession.

Before creating a copy of SoftInputWindow for VoiceInteractionSession,
this CL aims to clean up lint errors and style nits as many as
possible, notably:

 * Addressing lint errors
 * Adding missing @Override
 * Making SoftInputWindow a final class
 * Making internal member fields private
 * Removing SoftInputWindow#{get,set}Gravity(), which are no longer used
 * Renaming SoftInputWindowState to WindowState

There should be no observable behavior change in this CL.

Bug: 192412909
Test: prebuilts/checkstyle/checkstyle.py -f \
  frameworks/base/core/java/android/inputmethodservice/SoftInputWindow.java
Change-Id: I547489d3946bf523a59ada80f63cfd934e70c1f4