Use aconfig flag for desktop mode

Delays registration of DesktopModeLaunchParamsModifier to after
receiving onSystemReady() callback. Otherwise we are trying to access
the desktop windowing flag too early, before the flagging system is
initialized.

Bug: 304778354
Test: enable aconfig flag, check that desktop windowing is available

Change-Id: I5d743e34e943657f8485e5de50c660f67068660e