LocalSocketImpl.cpp: Set O_CLOEXEC on received FDs

Set O_CLOEXEC on received file descriptors. This prevents FD leakage
across an exec boundary. Please see the description of O_CLOEXEC in "man
2 open" for the reasons why this is useful.

Test: Device boots and no observed errors.
Change-Id: Id0d9c9d71efb25dacb26d5d98d93fc3b72c1b6ae