Remove upside-down portrait orientation.

I also realized the ROTATION_NN constants were misdocumented, they
actually align the opposite direction from the "orientation" angle
computed in the code (i.e. orientation=90 is left side up, ROTATION_90
is right side up).  I just changed the comment here to keep the change
minimal; I'll update the code to be more consistent outside of this
release branch.

Bug: 2549875
Change-Id: I79c4286be582a21e730b25a8a24f2e1137c6e4cb