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/include/camera/camera2/ICameraDeviceUser.h b/include/camera/camera2/ICameraDeviceUser.h
index c850924..619b161 100644
--- a/include/camera/camera2/ICameraDeviceUser.h
+++ b/include/camera/camera2/ICameraDeviceUser.h
@@ -133,6 +133,11 @@
      */
     virtual status_t        flush(/*out*/
                                   int64_t* lastFrameNumber = NULL) = 0;
+
+    /**
+     * Preallocate buffers for a given output stream asynchronously.
+     */
+    virtual status_t        prepare(int streamId) = 0;
 };
 
 // ----------------------------------------------------------------------------