Add aria-readonly to readonly cells

[amanmahajan7]

https://www.w3.org/TR/wai-aria-practices-1.2/#grid_roles_states_props

If the grid provides content editing functionality and contains cells that may have edit capabilities disabled in certain conditions, aria-readonly may be set true on cells where editing is disabled. If edit functions are disabled for all cells, aria-readonly may be set true on the grid element. Grids that do not provide editing functions do not include the aria-readonly attribute on any of their elements.