libbinder: shutdownAndWait based on # of in conns

shutdownAndWait no longer waits when it is called on a zero-thread
session (the session only needs to be deleted). However, more
importantly, we save a small amount of memory, and the log here is much
better (we can see how many incoming threads are stuck).

Bug: N/A
Test: binderRpcTest
Change-Id: I5924a82c33b4c3c2c970e6c6e86f1b8e2af074f5