Move selection of C++ STL into the build system.

Preparing for migration from stlport to libc++. STL selection is done
with LOCAL_CXX_STL (valid values are default, none, libc++,
libc++_static, stlport, stlport_static, bionic).

The selection of the STL is as follows:

    if LOCAL_CXX_STL == 'default'
      ifdef LOCAL_SDK_VERSION
        Use whatever STL the other NDK options have selected.
      else
        Use bionic's libstdc++ for target, GNU libstdc++ for host. This
        is compatible with the existing build options.
      endif
    else
      if LOCAL_CXX_STL == 'stlport'
        Use stlport.
      else if LOCAL_CXX_STL == 'libc++'
        Use libc++.
      else if LOCAL_CXX_STL == ''
        Don't use any STL.
      endif
    endif

Bug: 15193147
Change-Id: If712ba0ae7908d8147a69e29da5c453a183d6540