Window Manager Flag Migration (1/n)

Creates 2 APIs for apps to control system bars:

setSystemBarsAppearance - controls the appearance of system bars

setSystemBarsBehavior - determines how the bars behave when being hidden
                        by the application

Bug: 118118435
Test: manual - check if the appearance and behavior can be printed as
               expected with dumpsys
      (plan to add automatic tests later)
Change-Id: Ia6fe5f594960f218ff4e94ff481f514660dd5066