Mark GrantPermissionsViewModel callback as nullable

See: Ic6258b39fdbd7a7280c65f9e39e12c923dbcee48

Bug: 322010736
Change-Id: Ic879284a7480aa9bdf35bb8f1f9efff8622909d3
Merged-In: I7fa6f428512cf94518585b2fd5a4e0d9685923d7
Test: presubmit
1 file changed