Do not force desktop mode on a rear positioned display

Also removes check that would remove system decorations from
any rear positioned display, regardless if desktop mode
is being forced or not.

Fixes: 346103150
Test: Enable "Force desktop mode", then start rear display
      presentation. Observe no crash
Change-Id: I0ced425aad2bffd4db1021aa60f4969ad3f2c1bb