Address grids column headers accessibility issues - active descendant, what is announced by SRs - 19.2.x

[ddaribo]

Closes #15962

• 1. FIX aria-activedescendant on headers + TEST ❗(likely a regression)
• 2. Add aria-sort on column headers + update SPEC
• 3. Icons should be aria-hidden (i.e. not read by SR/UA) - separate PR
• 4. For a virtualized grid, cell row/col indexes are not announced properly

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them