[2/N] Desks: Add multi-desk data structures

This change adds basic new data structures to prepare for multi-desk
support and renames some existing ones to better align with multi-desks:
1) Adds DesktopDisplay: contains all desks as a well as the active one
2) Renames DesktopTaskData to Desk

Also moves |desktopTaskDataByDisplayId| behind a DesktopData interface
to allow swapping the implementation with one that supports
multiple desks when the flag is enabled, and refactors all callsites to
use the interface instead.

Finally, adds some basic functions to DesktopRepository to create,
remove and mark a desk as active.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 389960283
Test: with flag disabled, no changes
Test: atest WMShellUnitTests
Change-Id: I64383b25af627d78762dbd375b87480e8da2bc21