Refactor computeScreenConfigurationLocked and more.

- Recognizing that computeScreenConfigurationLocked() only returned
false if mDisplayReady was false allows multiple code
simplifications.
- Make computeNewConfigurationLocked() always return a non-null
value.
- Remove unused Session parameter from removeWindowInnerLocked().
- Just allocate mForceRemoves one time.
- Since ADJUST_WALLPAPER_VISIBILITY_CHANGED was unused have
adjustWallpaperWindowsLocked() return a boolean.
- Remove unused long parameter from handleNotObscuredLocked().

Change-Id: Ifd15736b163ab6e989f8ca32cf8f94b316386216