test_vendor: Use pipe2 to set pipe flags

Simplify the code to fix a typo.  F_SETFD should have been
F_SETFL.

Test: run unit tests

out/host/linux-x86/nativetest/test-vendor_test_host/test-vendor_test_host \
        --gtest_filter=AsyncManagerSocketTest.*

Change-Id: I215381d6e130428acae7d73d80fefc97cbf64651