DefaultPermissionGrantPolicy: Check feature flag when parsing permissions.

This ensures that we can add new pre-granted permissions to the list in a way that is flag protected.

Test: Verified that pre-granted permission reflected flag value. More details in https://docs.google.com/document/d/19Yg1s1dRahnSfiRMqDukcT4XD3fD8tVZCdSRaVyKlyY/edit?tab=t.1cxhi22djihy
Bug: 377545213
Flag: EXEMPT Adding support for permissions to be added by flag.
Change-Id: I111c5a38b1692ce3d10ebc3e879b6f5c92f6f50d