Correct nullability annotation in onActivityResult()

The intent argument inside Activity#onActivityResult should be @Nullable
instead of @NonNull. That is because apps can set the result via
Activity#setResult(int resultCode), which leads to a null result intent.

Bug: 293467489
Test: m and flash
Change-Id: I962c477cde556803277483d58423292d4f70e0c3