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/time.h b/libc/include/time.h
index a7a6fe5..0db14ff 100644
--- a/libc/include/time.h
+++ b/libc/include/time.h
@@ -78,7 +78,7 @@
 char* strptime_l(const char* __s, const char* __fmt, struct tm* __tm, locale_t __l) __strftimelike(2) __INTRODUCED_IN(28);
 
 size_t strftime(char* __buf, size_t __n, const char* __fmt, const struct tm* __tm) __strftimelike(3);
-#if __ANDROID_API__ >= __ANDROID_API_L__
+#if __ANDROID_API__ >= 21
 size_t strftime_l(char* __buf, size_t __n, const char* __fmt, const struct tm* __tm, locale_t __l) __strftimelike(3) __INTRODUCED_IN(21);
 #else
 // Implemented as static inline before 21.