Fix adb leaking file descriptors to forked processes

accept() creates a new file descriptor that should be closed on exec so
that forked processes don't keep a fd opened on the socket.

This also fixes b/3297070 where adb hangs after running adb on the
target.

Change-Id: I8df511289e5549ae49b4824c9dfb71a3bf85eae8
4 files changed