libbinder: BBinder descriptor

Two reasons:
- expose information to client (helps debugging)
- client code re-fetches blank descriptor

libbinder_ndk always requires a descriptor and
JavaBBinder overrides this method, so there is no
app compat concern.

Bug: N/A (though noticed in development of b/238777741)
Test: binderLibTest
Change-Id: Id31ac631b929350b97ab865c720f1dc1dfa26d2b