libbinder: add log for dropped extension

In the NDK/Rust backend, it's pretty easy to create a binder, attach
an extension to it, and then immediately destroy that binder.

Here, I add a log when this happens and add a unit test for BBinder
which triggers this path (it's never expected to be hit in practice
since binders aren't created without being sent).

Bug: N/A
Test: binderUnitTests
Change-Id: Id2b51396b027530e3fad767f698a3b2cecbfa98d