[Accessibility API] Add flag for mutli-node selection API.

This CL will add a flag for the API defined here:
go/android-a11y-multi-node-selection


Flag: android.view.accessibility.a11y_selection_api
Test: Adding flag, tests not required.
Bug: 362782536
Change-Id: I7e10504ac5c6999c2ba1376d39b114c072cf30c3