Migrate libmediandk to ndk_library.

Test: make native
Bug: http://b/30465923
Change-Id: Id62eac2d5cc518465445d87ce204fa918f0ddb68
diff --git a/include/ndk/NdkMediaDrm.h b/include/ndk/NdkMediaDrm.h
index 3c312a9..9dd6283 100644
--- a/include/ndk/NdkMediaDrm.h
+++ b/include/ndk/NdkMediaDrm.h
@@ -27,14 +27,17 @@
 #ifndef _NDK_MEDIA_DRM_H
 #define _NDK_MEDIA_DRM_H
 
+#include <stdbool.h>
+#include <stdint.h>
+#include <sys/cdefs.h>
+
 #include "NdkMediaError.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <stdint.h>
-#include <stdbool.h>
+#if __ANDROID_API__ >= 21
 
 struct AMediaDrm;
 typedef struct AMediaDrm AMediaDrm;
@@ -448,6 +451,8 @@
         const char *macAlgorithm, uint8_t *keyId, const uint8_t *message, size_t messageSize,
         const uint8_t *signature, size_t signatureSize);
 
+#endif /* __ANDROID_API__ >= 21 */
+
 #ifdef __cplusplus
 } // extern "C"
 #endif