adb: don't dup local socket fd.

SIGHUP handling depends on the local socket fd being fully closed in
order to trigger an event on its counterpart. Release the local socket
fd inside of Subprocess when returning it to ensure this.

Functionally, a cherry-pick of internal commit 42afe203.

Bug: http://b/29565233
Change-Id: I13b17bcddf0b396a5f4880f9e410fbbf24d9370d