Remove ParcelFileDescriptor hidden API usage from IpSecService

IpSec service is going to be a part of Connectivity mainline module
and ParcelFileDescriptor(FileDescriptor) is a hidden API that cannot
be used by mainline module.

Therefore, use ParcelFileDescriptor.dup(FileDescriptor) to get a
ParcelFileDescriptor instead.

Bug: 204153604
Test: FrameworksNetTests
Change-Id: I0ccabdfc5060f4b635e9a7009a67fbd5d32002fd