libbinder: do not destruct SM global

Let linux clean it up, since this may be accessed by some threads after
the main thread is destructed.

Bug: 154507808 # longterm
Bug: 156785633

Test: w/ repro which calls defaultServiceManager off main thread
Change-Id: Ic109f4bdca3893e6b0b192ac27f3ff03ada6f9e2