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/device2/Camera2Device.h b/services/camera/libcameraservice/device2/Camera2Device.h
index 9972606..fd1240a 100644
--- a/services/camera/libcameraservice/device2/Camera2Device.h
+++ b/services/camera/libcameraservice/device2/Camera2Device.h
@@ -84,6 +84,9 @@
             buffer_handle_t *buffer, wp<BufferReleasedListener> listener);
     // Flush implemented as just a wait
     virtual status_t flush(int64_t *lastFrameNumber = NULL);
+    // Prepare is a no-op
+    virtual status_t prepare(int streamId);
+
     virtual uint32_t getDeviceVersion();
     virtual ssize_t getJpegBufferSize(uint32_t width, uint32_t height) const;