Unset last orientation for fixed orientation transient launch

The commit 7c6f6b80 uses the requested orientation. That is fine
if the next app has different requested orientation. But if the
next app is still the transient launch app itself, then no change
will be detected by DisplayRotation#updateOrientation, and then
the display stay in previous rotation.

Change to use UNSET so it is similar to force update for the next
time calling updateOrientation().

Bug: 216169338
Test: atest TransitionTests#testDeferRotationForTransientLaunch
Change-Id: Id467d1910b32d8699fb34e7fdc04c91c7b450c71