libbinder: avoid re-registering callback

This code assumes that it is only registered once, and so in some cases,
the service wasn't shutting down.

Bug: 140310064
Test: aidl_lazy_test
Change-Id: I19474e105b4c4b3669499ac71c8cf22b67a0176b
Merged-In: I19474e105b4c4b3669499ac71c8cf22b67a0176b