Move more code into ScreenshotViewProxy

- viewTreeObserver (only used to postpone the startAnimation call until
  after the view has loaded in onPreDraw)
- setDefaultTimeout (this can't be anything other than the
  SCREENSHOT_CORNER_DEFAULT_TIMEOUT_MILLIS)

Test: manual, atest
Flag: NONE
Change-Id: I520affe2b957fc064598f295cfec341f60318c8c