Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/libc/include/termios.h b/libc/include/termios.h
index c17794c..49ffcd2 100644
--- a/libc/include/termios.h
+++ b/libc/include/termios.h
@@ -35,7 +35,7 @@
 
 __BEGIN_DECLS
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 // Implemented as static inlines before 21.
 speed_t cfgetispeed(const struct termios*) __INTRODUCED_IN(21);
 speed_t cfgetospeed(const struct termios*) __INTRODUCED_IN(21);