Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/libc/include/wchar.h b/libc/include/wchar.h
index 2ee6c69..d3e9f5c 100644
--- a/libc/include/wchar.h
+++ b/libc/include/wchar.h
@@ -132,7 +132,7 @@
 int               wprintf(const wchar_t *, ...);
 int               wscanf(const wchar_t *, ...);
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 long long wcstoll_l(const wchar_t*, wchar_t**, int, locale_t) __INTRODUCED_IN(21);
 unsigned long long wcstoull_l(const wchar_t*, wchar_t**, int, locale_t) __INTRODUCED_IN(21);
 long double wcstold_l(const wchar_t*, wchar_t**, locale_t) __INTRODUCED_IN(21);