Initial implementation of the AddDesktopButton

Add the AddDesktopButton to the view hierarchy behind the flags.

Flag: com.android.window.flags.enable_multiple_desktops_frontend
      com.android.window.flags.enable_multiple_desktops_backend
Bug: 382057498
Test: Manual
Change-Id: I5748d8c18d8f5204fa1fb0717c4a8098bfde1537
5 files changed