Add Flicker test for drag-to-maximize on desktop-first mode

This CL adds a flicker test and functional test for drag-to-maximize on
desktop-first mode.
Connecting an external display enables the desktop-first mode which
changes the consequence of dragging a window to the top edge of the
screen from “exit desktop” to “maximize”.

Flag: com.android.window.flags.enable_desktop_first_based_drag_to_maximize
Bug: 413215717
Test: MaximizeAppWindowWithDragToTopDragZoneInDesktopFirstLandscape
Change-Id: I2bb0218f0dc2288b82af8feb2d4044352d71e318