Update docs for YV12 format and camera preview callbacks.

- Define stride for YV12 when using it for preview callbacks

- Include equations for calculating stride and start indexes of Y, U,
  and V planes for YV12.

- Add more cross-references so that equations are easier to find.

Bug: 6330501
Change-Id: I85a78757ec767d08173b9fe714adb715835244b4