Set window to transparent when screenshotting layer.

Set the screenshot to transparent instead of black. This allows the
system to draw a color behind without having to crop out the area that
may not be drawn into.

Bug: 76442549
Test: When going to recents, area not drawn from layers is now
transparent instead of black.
Test: Transaction_test#CaptureTransparent

Change-Id: Id535eb753d3d1cae82658cd33423ce588aaa62f8