Crop surface to the content size during drag-resizing

During drag-resizing, a fullscreen-sized surface is reused to
optimize reallocations, which can leave outdated app content on the
surface when resizing to a smaller size. This outdated app content
can then be briefly exposed to the user when drag-resizing to a
larger size until the app draws again and fills the new size.
To "clear" this garbage content, this change crops the content
directly from the client side during drag-resizing. The "gap" between
the cropped app content and the larger task that's in sync with the
drag-gesture is now the solid background color of the task surface
instead of the former garbage app content.

Bug: 270202228
Test: quickly drag-resize a freeform task to a smaller and then a
larger size, verify that the "gap" exposed while the app draw catches
up to the resize gesture is now a solid color instead of artifacts
from a previous app draw.

Change-Id: Ibc3ca2a8f5b83b3fef2fe4832f93ca52d3b156a8