Add IDesktopMode multiple desktops APIs

Add two new APIs for multiple desktops:
- `createDesk()`.
- `activateDesk()`.
Currently these APIs are not hooked up yet. This will be done in
follow-up CLs.

Also removes the unused `getVisibleTaskCount()` API.

Bug: 390715986
Test: m
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Change-Id: If5b28c54c0f09caa36fc70913bbcbd2b1faf1686