Camera: BufferManager: Don't attach/detach on every frame

- Maintain separate count of attached buffers
- Only attach when new buffers need to be allocated
- Only detach when a buffer needs to be freed
- Fix missing notification initializations
- Remove warning that's always logged

Bug: 28695173
Change-Id: I38e997fa1e69c2b8743e43eed31a6a08a6f9cd7a