EMFILE exception in system_server due to socket fd leaks

NativeCrashListener creates a socket connection with debuggerd.
When native crash is observed, NativeCrashListener gets new socket
file descriptor from debuggered. This socket is never closed.
Multiple native crashes are causing "Too many files opened" EMFILE
exception in system_server.

CRs-Fixed: 545566

Change-Id: Idb12e87a4f91c978169435ce95abe11b40c6d0dc