SpAIBinder: allow comparison with pointers

It's pretty annoying to need to manually de-reference
this. I fixed it on sp<> a long, long time ago, so I'm
not sure why I never fixed it when I created this type.

Bug: N/A
Test: libbinder_ndk_unit_test
Change-Id: I3143dde25ccc043ee60d85db894eb6e6aedf5690