Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/libc/include/time.h b/libc/include/time.h
index 37246b4..9fde77e 100644
--- a/libc/include/time.h
+++ b/libc/include/time.h
@@ -77,7 +77,7 @@
 char* strptime(const char*, const char*, struct tm*);
 size_t strftime(char*, size_t, const char*, const struct tm*);
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 size_t strftime_l(char*, size_t, const char*, const struct tm*, locale_t) __INTRODUCED_IN(21);
 #else
 // Implemented as static inline before 21.