Fix a tracked reference leak in dvmGenerateProxyClass.

Bug: https://code.google.com/p/android/issues/detail?id=43919
Change-Id: Ib5c217368ba75f4422d0c932c2c67daaaeddea4b
Use execv from Libcore

Change-Id: Idc862adef2a95636c85992e1954651b4e0bcaf16