Add new APIs AMessage::(set|find)Buffer to make it safer to pass

ABuffer objects through messages.

Change-Id: I9f8b4e4c4767d0d70a0105e0c0813b754379b49d
diff --git a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
index 6eb0d07..4c65b65 100644
--- a/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/RTSPSource.cpp
@@ -218,10 +218,8 @@
             CHECK(msg->findSize("trackIndex", &trackIndex));
             CHECK_LT(trackIndex, mTracks.size());
 
-            sp<RefBase> obj;
-            CHECK(msg->findObject("accessUnit", &obj));
-
-            sp<ABuffer> accessUnit = static_cast<ABuffer *>(obj.get());
+            sp<ABuffer> accessUnit;
+            CHECK(msg->findBuffer("accessUnit", &accessUnit));
 
             int32_t damaged;
             if (accessUnit->meta()->findInt32("damaged", &damaged)