Migrate libmediandk to ndk_library.

Test: make native
Bug: http://b/30465923
Change-Id: Id62eac2d5cc518465445d87ce204fa918f0ddb68
diff --git a/include/ndk/NdkMediaCrypto.h b/include/ndk/NdkMediaCrypto.h
index 90374c5..9236765 100644
--- a/include/ndk/NdkMediaCrypto.h
+++ b/include/ndk/NdkMediaCrypto.h
@@ -28,6 +28,7 @@
 #ifndef _NDK_MEDIA_CRYPTO_H
 #define _NDK_MEDIA_CRYPTO_H
 
+#include <sys/cdefs.h>
 #include <sys/types.h>
 #include <stdbool.h>
 
@@ -35,6 +36,8 @@
 extern "C" {
 #endif
 
+#if __ANDROID_API__ >= 21
+
 struct AMediaCrypto;
 typedef struct AMediaCrypto AMediaCrypto;
 
@@ -48,6 +51,7 @@
 
 void AMediaCrypto_delete(AMediaCrypto* crypto);
 
+#endif /* __ANDROID_API__ >= 21 */
 
 #ifdef __cplusplus
 } // extern "C"