Fix issue around Encryption during Pairing

* ECC_PointMult erases private key argument, pass copy instead of
original value.
* Specify Z parameter for ECC Points

This fixes issues where LE Secure connection pairing between old and new
stack was failing due to DHKey mismatch

Test: manual pairing between phone running new and old stack
Bug: 147086339
Change-Id: Ied3404c2eb2ca55b717042ea099e1aefed1c3e8d