Restore __ANDROID_API__ guards for camera/media headers

Partial revert of "Add __INTRODUCED_IN to audio/camera/media headers.",
commit 85a41536a4885b12120ddafb4d0b3fee8a8613b6.

Restore the API guards where they used to exist. I *could* add
__ANDROID_API__ around only the function declarations, which is what the
versioner would do, but the function declarations are mixed up with the
types and constants, so I'd need to add a bunch of __ANDROID_API__ checks
or reorganize the headers.

NdkMediaFormat.h and AAudio.h still have __INTRODUCED_IN symbols that
aren't guarded by __ANDROID_API__. Maybe those files should be reorganized
so they don't alternate between APIs so much.

Test: builds
Bug: http://b/111668906
Bug: http://b/113052379
Change-Id: I757a6097f7840ea50e2fc92db7defef493097672
Merged-In: I757a6097f7840ea50e2fc92db7defef493097672
(cherry picked from commit 0c9470cec23a62876c398bc6ab9ac7e1588e2bba)
diff --git a/media/ndk/include/media/NdkMediaMuxer.h b/media/ndk/include/media/NdkMediaMuxer.h
index 75c70ed..7393867 100644
--- a/media/ndk/include/media/NdkMediaMuxer.h
+++ b/media/ndk/include/media/NdkMediaMuxer.h
@@ -53,6 +53,8 @@
     AMEDIAMUXER_OUTPUT_FORMAT_WEBM   = 1,
 } OutputFormat;
 
+#if __ANDROID_API__ >= 21
+
 /**
  * Create new media muxer
  */
@@ -121,6 +123,8 @@
         size_t trackIdx, const uint8_t *data,
         const AMediaCodecBufferInfo *info) __INTRODUCED_IN(21);
 
+#endif /* __ANDROID_API__ >= 21 */
+
 __END_DECLS
 
 #endif // _NDK_MEDIA_MUXER_H