Add experimental camera session prepare API.

Bug: 18949148
Change-Id: I8f73e68ea2e3acc60d98954106f364d13f439a82
diff --git a/services/camera/libcameraservice/api2/CameraDeviceClient.h b/services/camera/libcameraservice/api2/CameraDeviceClient.h
index 1f8b39d..0b73f6c 100644
--- a/services/camera/libcameraservice/api2/CameraDeviceClient.h
+++ b/services/camera/libcameraservice/api2/CameraDeviceClient.h
@@ -114,6 +114,9 @@
     // Tear down stream resources by freeing its unused buffers
     virtual status_t      tearDown(int streamId);
 
+    // Prepare stream by preallocating up to maxCount of its buffers
+    virtual status_t      prepare2(int maxCount, int streamId);
+
     /**
      * Interface used by CameraService
      */