Make IpClientIntegrationTest pass on rvc-dev.

On rvc-dev, IpClientIntegrationTest fails almost all the time
because the tap interface disappears before the tests can run.
This is due to the ParcelFileDescriptor finalizer, which
helpfully closes the fd stored in the object even if it is in
use elsewhere.

Stash the ParcelFileDescriptor itself in the test class, instead
of the actual FileDescriptor which everything uses, in order to
ensure that the garbage collector does not finalize it.

With this change, IpClientIntegrationTest#testRaRdnss (just an
arbitrary test case in that class) goes from 10/10 failures to
10/10 passes.

Bug: 152723363
Test: atest NetworkStackNextIntegrationTests:IpClientIntegrationTest#testRaRdnss
Change-Id: I1015e7893ba6af74876665826960e8fcc1711476