Misc cleanups and simplifications

TakeScreenshotService
  migrated to Handler.Callback
  skip posting to handler for binder call

ScreenshotController
  create WindowContext from DisplayContext
  simplified configuration change checking

Test: adb shell input SYSRQ
      change system dark mode, repeat
      open overview, tap screenshot action

Change-Id: I18fc23f9e00cc0a455335d6de6975ded669b6629