RpcServer::setupSocketServer prevent race

If two threads calls setup*Server, they may race.
Make sure one of them fail.

setupSocketServer calls into setupExternalServer to
double check that no other thread was setting up
server in the mean time to prevent TOCTOU.

This change also ensures that setupExternalServer is the
only place where mServer is set, and it is guarded by a
lock.

Test: binderRpcTest

Change-Id: Iadd3585b9cb4aa157bae072943f1bd47896a7a77