Fix build from broken makefile.

LOCAL_NDK_STL_VARIANT is a no-op if LOCAL_SDK_VERSION is not set, and
this project cannot actually be built with the NDK (it uses platform
only libraries like cutils).

There's no reason (I hope) to be using stlport in the library anyway,
so just let them use all of libc++.

Change-Id: Ia917dbaf347cc438990386d21d95b8b73cd34614