Stop using the __ANDROID_API_x__ constants.

Historically we've made a few mistakes where they haven't matched the
right number. And most non-Googlers are much more familiar with the
numbers, so it seems to make sense to rely more on them. Especially in
header files, which we actually expect real people to have to read from
time to time.

Test: treehugger
Change-Id: I0d4a97454ee108de1d32f21df285315c5488d886
diff --git a/libc/include/sys/socket.h b/libc/include/sys/socket.h
index e54dd65..e2acab7 100644
--- a/libc/include/sys/socket.h
+++ b/libc/include/sys/socket.h
@@ -112,7 +112,7 @@
    ? (struct cmsghdr*) (msg)->msg_control : (struct cmsghdr*) NULL)
 #define CMSG_OK(mhdr, cmsg) ((cmsg)->cmsg_len >= sizeof(struct cmsghdr) &&   (cmsg)->cmsg_len <= (unsigned long)   ((mhdr)->msg_controllen -   ((char*)(cmsg) - (char*)(mhdr)->msg_control)))
 
-#if __ANDROID_API__ >= __ANDROID_API_L__
+#if __ANDROID_API__ >= 21
 struct cmsghdr* __cmsg_nxthdr(struct msghdr* __msg, struct cmsghdr* __cmsg) __INTRODUCED_IN(21);
 #else
 /* TODO(danalbert): Move this into libandroid_support. */
@@ -127,7 +127,7 @@
   }
   return ptr;
 }
-#endif /* __ANDROID_API__ >= __ANDROID_API_L__ */
+#endif
 
 #define SCM_RIGHTS 0x01
 #define SCM_CREDENTIALS 0x02