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/Camera3OutputStream.h b/services/camera/libcameraservice/device3/Camera3OutputStream.h
index 12b2ebb..513b695 100644
--- a/services/camera/libcameraservice/device3/Camera3OutputStream.h
+++ b/services/camera/libcameraservice/device3/Camera3OutputStream.h
@@ -99,7 +99,7 @@
 
     virtual status_t configureQueueLocked();
 
-    virtual status_t getEndpointUsage(uint32_t *usage);
+    virtual status_t getEndpointUsage(uint32_t *usage) const;
 
 }; // class Camera3OutputStream