Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/libc/include/sys/stat.h b/libc/include/sys/stat.h
index cc9dd91..f961cf8 100644
--- a/libc/include/sys/stat.h
+++ b/libc/include/sys/stat.h
@@ -169,7 +169,7 @@
 
 #if defined(__BIONIC_FORTIFY)
 
-#if __ANDROID_API__ >= 18
+#if __ANDROID_API__ >= __ANDROID_API_J_MR2__
 __BIONIC_FORTIFY_INLINE
 mode_t umask(mode_t mode) {
 #if !defined(__clang__)
@@ -182,11 +182,11 @@
 #endif
   return __umask_chk(mode);
 }
-#endif /* __ANDROID_API__ >= 18 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_J_MR2__ */
 
 #endif /* defined(__BIONIC_FORTIFY) */
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 int mkfifo(const char*, mode_t) __INTRODUCED_IN(21);
 #else
 // Implemented as a static inline before 21.