Add and use constants for the Android API levels.

Test: bionic tests still pass
Change-Id: If1b619cfc9db1d3f5d91f14f2ace71058dca0c4a
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 130ede6..1e239cd 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -169,7 +169,7 @@
 
 off64_t lseek64(int __fd, off64_t __offset, int __whence);
 
-#if defined(__USE_FILE_OFFSET64) && __ANDROID_API__ >= 21
+#if defined(__USE_FILE_OFFSET64) && __ANDROID_API__ >= __ANDROID_API_L__
 int truncate(const char* __path, off_t __length) __RENAME(truncate64) __INTRODUCED_IN(21);
 ssize_t pread(int __fd, void* __buf, size_t __count, off_t __offset) __RENAME(pread64)
   __INTRODUCED_IN(12);
@@ -206,7 +206,7 @@
 
 int acct(const char* __filepath);
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 int getpagesize(void) __INTRODUCED_IN(21);
 #else
 static __inline__ int getpagesize(void) {
@@ -284,7 +284,7 @@
 
 #if defined(__BIONIC_FORTIFY)
 
-#if __ANDROID_API__ >= 24
+#if __ANDROID_API__ >= __ANDROID_API_N__
 __BIONIC_FORTIFY_INLINE
 char* getcwd(char* buf, size_t size) {
     size_t bos = __bos(buf);
@@ -315,7 +315,7 @@
 
     return __getcwd_chk(buf, size, bos);
 }
-#endif /* __ANDROID_API__ >= 24 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
 
 #if defined(__USE_FILE_OFFSET64)
 #define __PREAD_PREFIX(x) __pread64_ ## x
@@ -323,7 +323,7 @@
 #define __PREAD_PREFIX(x) __pread_ ## x
 #endif
 
-#if __ANDROID_API__ >= 23
+#if __ANDROID_API__ >= __ANDROID_API_M__
 __BIONIC_FORTIFY_INLINE
 ssize_t pread(int fd, void* buf, size_t count, off_t offset) {
     size_t bos = __bos0(buf);
@@ -373,7 +373,7 @@
 
     return __pread64_chk(fd, buf, count, offset, bos);
 }
-#endif /* __ANDROID_API__ >= 23 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_M__ */
 
 #if defined(__USE_FILE_OFFSET64)
 #define __PWRITE_PREFIX(x) __pwrite64_ ## x
@@ -381,7 +381,7 @@
 #define __PWRITE_PREFIX(x) __pwrite_ ## x
 #endif
 
-#if __ANDROID_API__ >= 24
+#if __ANDROID_API__ >= __ANDROID_API_N__
 __BIONIC_FORTIFY_INLINE
 ssize_t pwrite(int fd, const void* buf, size_t count, off_t offset) {
     size_t bos = __bos0(buf);
@@ -431,9 +431,9 @@
 
     return __pwrite64_chk(fd, buf, count, offset, bos);
 }
-#endif /* __ANDROID_API__ >= 24 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
 
-#if __ANDROID_API__ >= 21
+#if __ANDROID_API__ >= __ANDROID_API_L__
 __BIONIC_FORTIFY_INLINE
 ssize_t read(int fd, void* buf, size_t count) {
     size_t bos = __bos0(buf);
@@ -458,9 +458,9 @@
 
     return __read_chk(fd, buf, count, bos);
 }
-#endif /* __ANDROID_API__ >= 21 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_L__ */
 
-#if __ANDROID_API__ >= 24
+#if __ANDROID_API__ >= __ANDROID_API_N__
 __BIONIC_FORTIFY_INLINE
 ssize_t write(int fd, const void* buf, size_t count) {
     size_t bos = __bos0(buf);
@@ -487,9 +487,9 @@
 
     return __write_chk(fd, buf, count, bos);
 }
-#endif /* __ANDROID_API__ >= 24 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_N__ */
 
-#if __ANDROID_API__ >= 23
+#if __ANDROID_API__ >= __ANDROID_API_M__
 __BIONIC_FORTIFY_INLINE
 ssize_t readlink(const char* path, char* buf, size_t size) {
     size_t bos = __bos(buf);
@@ -539,7 +539,7 @@
 
     return __readlinkat_chk(dirfd, path, buf, size, bos);
 }
-#endif /* __ANDROID_API__ >= 23 */
+#endif /* __ANDROID_API__ >= __ANDROID_API_M__ */
 
 #endif /* defined(__BIONIC_FORTIFY) */