Always create grant structures when persistable.

Certain apps may already hold permissions to an underlying provider,
but they expect APIs like takePersistableUriPermission() and
getPersistedUriPermissions() to work when a permission grant was
extended with FLAG_GRANT_PERSISTABLE_URI_PERMISSION.

Test: builds, boots
Bug: 31239684
Change-Id: I4b21c57956b70133ecadb50d0d3ee339f41e2260