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/NuPlayer.cpp b/media/libmediaplayerservice/nuplayer/NuPlayer.cpp
index 04415cd..dec1c08 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayer.cpp
+++ b/media/libmediaplayerservice/nuplayer/NuPlayer.cpp
@@ -768,7 +768,7 @@
          mediaTimeUs / 1E6);
 #endif
 
-    reply->setObject("buffer", accessUnit);
+    reply->setBuffer("buffer", accessUnit);
     reply->post();
 
     return OK;
@@ -793,10 +793,8 @@
         return;
     }
 
-    sp<RefBase> obj;
-    CHECK(msg->findObject("buffer", &obj));
-
-    sp<ABuffer> buffer = static_cast<ABuffer *>(obj.get());
+    sp<ABuffer> buffer;
+    CHECK(msg->findBuffer("buffer", &buffer));
 
     int64_t &skipUntilMediaTimeUs =
         audio