Add DesktopModeStatus.enableMultipleDesktops()

This convenience method will be used in launhcer to check that the
device supports multiple desktops and both backend and frontend flags
are enabled.

Bug: 388527943
Test: m
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Change-Id: I374efcb272f5540b28fd333539f7f4df22852a92