Avoid fd to be closed by ParcelFileDescriptor's finalizer

testDropPacketToVpnAddress is flaky since ParcelFileDescriptor's
finalizer closes fd and makes fd invalid.
This CL fixes this issue by keeping the reference to
ParcelFileDescriptor.

Bug: 334644322
Test: VpnTest
Change-Id: Ib8f094725b39ac5824cd9a62918efe51f86e5cb1
1 file changed