libc++_static isn't a proper value for APP_STL, it should be c++_static here.
NDK_TOOLCHAIN_VERSION has no effect when set inside Android.mk, it should be set inside Application.mk
LOCAL_CLANG is a variable used inside system modules from AOSP, not when using the NDK.
Since you're setting APP_STL as c++_static, the NDK toolchain will correctly tell the linker what lib to use, you shouldn't add LOCAL_LDLIBS := -lc++_static.
Also, you set APP_ABI to only armeabi-v7a, is it on purpose ? Android runs on other architectures as well and you'll get better performance on these if you also compile your libraries accordingly. You can either set APP_ABI to all or to a list of architectures armeabi-v7a x86...