Un-hide ROTATION_ANIMATION_SEAMLESS.

For N the Camera used ROTATION_ANIMATION_JUMPCUT
which was reinterpreted as seamless rotation if possible.
However this ended up being non ideal in app transition
scenarios where the rotation animation replaces the app
transition. Here we also can't seamlessly rotate, so the
window just suddenly appears with jumpcut. In fact we want
crossfade as a fallback animation and this is what distinguishes
ROTATION_ANIMATION_JUMPCUT and SEAMLESS. The problem was patched
over in N by having double-tap gestures fire camera intents
with a SEAMLESS ActivityOptions.rotationAnimationHint,
but it was too late to un-hide for consumption by apps. While here,
clean up some documentation that doesn't really parse.

Test: Manual, try the RotationAnimation activity in ApiDemos sample.
Bug: 28838855
Bug: 28823590
Change-Id: I17b5e9fa967df8ade0943337cd7b6c3956d3a499