Define a flicker test for no-desktop-task-limit

This CL adds a flicker test to open many apps on a device without
desktop task limit.

Bug: 376802328
Flag: EXEMPT add test
Test: atest WMShellFlickerTestsDesktopMode:com.android.wm.shell.flicker.OpenUnlimitedApps
Change-Id: If5488984f42d57417f23d7e55c3f5411013a7acd