JDWP: fix referenceTypeID read/write handling

JDWP specification says that objectID and referenceTypeID can have
different sizes. But the implementation incorrectly used to treat
referenceTypeID like objectID. This CL fixes that.

Bug: 65869324
Test: art/tools/run-jdwp-tests.sh '--mode=host' '--variant=X64'
Change-Id: Ic0f510a8b01b9b3687303566100e27728fede462