Properly use futures to wait on API result in SetPermissionGrantState.

Test: atest android.devicepolicy.cts.PermissionGrantTest with flag on/off
Flag: android.app.admin.flags.set_permission_grant_state_coexistence
Bug: 370472975
Bug: 336297680
Change-Id: I1f44d3de6aa7fbf61c85fe8e147fb381870def4a