Set close-on-exec flag for bugreportz socket in dumpstate

dumpstate triggers lshal child processes, bugreportz connection socket
fd in dumpstate is passed on to these processes.

Set close-on-exec flag allows to close the socket connection from
dumpstate, even if child processes do not terminate.

Test: `adb bugreport` `adb bugreport BR.zip` works as expected
Bug: 140713872
Change-Id: I4aa07a60110da383110e2e9091c72403351ac3dc