make libprotobuf-cpp-* libs VNDK

libprotobuf-cpp-* libs are VNDK, However, since the '.vendor' prefix is
added before libprotobuf-cpp-* libs are added to my-*-libraries by
LOCAL_PROTOC_OPTIMIZE_TYPE, the clients are still linked against to the
platform version of the protobuf libs. As a result, link type error
occurs.

In order to avoid this, '.vendor' prefix is appended after
my-*-*libraries are all updated.

Test: normal build successfull
Test: add vendor_provided:true to libprotobuf-cpp-* and build successful
with BOARD_VNDK_VERSION=true

Change-Id: I1ed7978f03e0db56b4cbee86e286c0787f7a3afc