Use ParcelFileDescriptor instead of FileDescriptor in INetd.aidl

Stable aidl won't support FileDescriptor but ParcelFileDescriptor.
In order to migrate to stable aidl, replace all FileDescriptor in
INdetd.aidl.

Test: built, flashed, booted
      system/netd/tests/runtests.sh passes

Change-Id: I331626346959f127b4c1cb2ece33db37cb8dc550