[Accessibility API] Add flag for is required api.

This CL will add a flag for the API defined here:
go/required-fields-api-change-design-proposal

Flag: android.view.accessibility.a11y_is_required_api
Test: Adding flag, tests not required.
Bug: 362784916
Change-Id: I19de3b9c9a6c2b3ed0667014da42e16b1d9676f4
1 file changed