Introduce A11yService#takeScreenshotOfWindow()

This enables accessibility services to take a screenshot of
a window without any overlay windows drawn on top, which is
beneficial for ML-based understanding of the visual content
in the window.

Bug: 249830826
Test: atest AccessibilityTakeScreenshotTest
Test: use in an a11y service, observe screenshots
      contain only content from the requested windows
      and no overlays
Change-Id: Ic886aeace9cb2984a472273858281b4f2c3a0e46