Make libc++ the default STL.

Previously, code defaulted to using libstdc++ (which was GNU libstdc++
on the host, or Bionic's "libstdc++" on the target). Now we default to
libc++. The exception for this is when using mingw, because we cannot
build libc++ for mingw.

Change-Id: I68726fa26cdcb6263f17c838d3e859debd57fae1
diff --git a/core/target_test_internal.mk b/core/target_test_internal.mk
index 50f4036..ed011d1 100644
--- a/core/target_test_internal.mk
+++ b/core/target_test_internal.mk
@@ -6,13 +6,12 @@
 
 LOCAL_C_INCLUDES += external/gtest/include
 
-my_test_libcxx := false
-ifndef LOCAL_SDK_VERSION
-ifeq (,$(TARGET_BUILD_APPS))
-ifneq ($(filter $(strip $(LOCAL_CXX_STL)),libc++ libc++_static),)
 my_test_libcxx := true
+ifdef LOCAL_SDK_VERSION
+my_test_libcxx := false
 endif
-endif
+ifneq ($(filter $(strip $(LOCAL_CXX_STL)),stlport stlport_static),)
+my_test_libcxx := false
 endif
 
 ifeq ($(my_test_libcxx),true)