Workaround for screenshot IME focusability

- Include back gestural insets into the touchable region for the
  screenshot window, and disable focusability when we receive an
  outside touch.  Also need to disallow screenshot windows from
  IME target calculations to prevent the IME from being made
  relative to the screenshot window.  Disabling focusability also
  means that apps will receive back once the user starts interacting
  outside the screenshot window.

Bug: 159460485
Bug: 151179149
Test: In 3button/gestural, take screenshot and check that back still
      dismisses screenshot.  Take another screenshot and tap on app
      that opens an IME and check that it opens the IME.  Once the
      user taps outside, ensure back goes to the app and not the
      screenshot window.

Change-Id: I1d2100d0640fec2efc36ac8afbf8ffaf900f3bd6