[CD][Alt+Tab] Add WMShell API to launch a fullscreen task from desktop

- These are the WMShell side of changes for flattening Alt+Tab KQS
- This will allow users to switch to a fullscreen app while in desktop mode using Alt+Tab.

Bug: 382769617
Test: atest WMShellUnitTests:DesktopTasksControllerTest and manually run CUJ
Flag: com.android.launcher3.enable_alt_tab_kqs_flatenning
Change-Id: I7de227d1a9398d7dfd268443b2d127e34f93b976