Use platform protobuf unless building with NDK.

The NDK protobuf library depends on the final target linking stlport
(since it is a static library). Since the platform stlport is going
away, we need to use a separate version of the protobuf library that
is compiled for the platform against libc++.

Note that this should be the case for _all_ libraries built with the
NDK. If a library needs to be used by both an NDK built final target
and a platform built final target, there should be both an NDK and
platform version of the library.

Bug: 15193147
Change-Id: I0ead61c2d1cd9d0248b304ab7d8682dedd6e8366