Mark GrantPermissions callback as nullable

The Intent taken by the consumer should have been marked nullable, but
wasn't.

Fixes: 322010736
Test: build
Relnote: androidx upgrades
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:196cec2e848943f29c6b2c4623ec5a80d8eb891a)
Merged-In: I7fa6f428512cf94518585b2fd5a4e0d9685923d7
Change-Id: I7fa6f428512cf94518585b2fd5a4e0d9685923d7