Camera2: Add prepare for output streams

The prepare call asynchronously pre-allocates buffers for a given
output stream, and then fires the onPrepared callback.

Not implemented for Camera2Device or used in Camera2Client.

Change-Id: I1cccdfff846dd6985133c591dbdceed823929ade
diff --git a/services/camera/libcameraservice/device3/Camera3InputStream.h b/services/camera/libcameraservice/device3/Camera3InputStream.h
index 7ba36c9..9f3de10 100644
--- a/services/camera/libcameraservice/device3/Camera3InputStream.h
+++ b/services/camera/libcameraservice/device3/Camera3InputStream.h
@@ -75,7 +75,7 @@
 
     virtual status_t configureQueueLocked();
 
-    virtual status_t getEndpointUsage(uint32_t *usage);
+    virtual status_t getEndpointUsage(uint32_t *usage) const;
 
 }; // class Camera3InputStream