Distinguish between force-enter-desktop and desktop-first

This CL distinguishes between force-enter-desktop and desktop-first.
Currently they are mixed up in the code base but they are different
(i.e., even if the display is in desktop-first mode, the policy is not
applied to every app (re)launch).
So this CL makes it clearer.

Please note that this change is a refactor and doesn’t include
functional changes.

Flag: EXEMPT - small refactor
Bug: 429130530
Test: existing tests pass
Change-Id: Ifb263e91fab5a237823fd56dbf1eb79adaa838ff