Add __ANDROID_API_FUTURE__.

Needed for checking conditions like `__ANDROID_API__ <
__ANDROID_API_FUTURE__` for providing inlines for things that have
not yet been included in any release.

Test: make checkbuild tests
Bug: None
Change-Id: Ibcddac5a538ede020a8cb65fca03194475121966
diff --git a/libc/include/android/api-level.h b/libc/include/android/api-level.h
index 001877b..6dd4c18 100644
--- a/libc/include/android/api-level.h
+++ b/libc/include/android/api-level.h
@@ -35,8 +35,12 @@
  * Magic version number for a current development build, which has
  * not yet turned into an official release.
  */
+#ifndef __ANDROID_API_FUTURE__
+#define __ANDROID_API_FUTURE__ 10000
+#endif
+
 #ifndef __ANDROID_API__
-#define __ANDROID_API__ 10000
+#define __ANDROID_API__ __ANDROID_API_FUTURE__
 #endif
 
 #endif /* ANDROID_API_LEVEL_H */