Test with C++1z.

The new clang drop isn't capable of building libc++ itself with C++1z
(I haven't investigate yet), but it seems we don't need that.

Test: make checkbuild && ./run_tests.py # sailfish
Bug: http://b/34740564
Change-Id: I0dfb578b1b04dd37108371deaa710681b0d88f4f
diff --git a/buildcmds/Android.mk b/buildcmds/Android.mk
index dd7a83e..c188b08 100644
--- a/buildcmds/Android.mk
+++ b/buildcmds/Android.mk
@@ -25,7 +25,7 @@
 LOCAL_CXX_STL := libc++
 LOCAL_C_INCLUDES := $(LOCAL_PATH)/../test/support
 LOCAL_CPPFLAGS := \
-    -std=c++14 \
+    -std=c++1z \
     -fsized-deallocation \
     -fexceptions \
     -UNDEBUG \