Revert "RotatingImageView invalidates"

This reverts commit 7f826ee74f57f8d7cfcc8a5294fd9048b154c175.
Invalidation happens in onRotationChanged

Last commit put the invalidate call in the wrong place (it should not be
invalidated unless the rotation does, in fact, change). Reverted and
fixed.
Knob listener and UI handling

- Now passes touch events to listener
- Handles case where scale is changed programmatically
- Minor doc clarification