Camera: Fix bufferManager interaction with discardFreeBuffers

- Revert ag/6652732 because now we have a BQ callback for
  discardFreeBuffers.
- Distinguish between detachBuffer and discardFreeBuffers.
  detachBuffer is called after acquireBuffer. For detachBuffer,
  both the handoutBuffers and attachedBuffers decrement by 1.
  Whereas for discardFreeBuffers, only the attachedBuffers
  decrements by 1, because the buffers are already released.
- Bubble up Camera3BufferManager errors to its client so that
  error conditions are reflected and easily caught by tests.

Flag: EXEMPT bugfix
Bug: 417670429
Test: Camera CTS
Change-Id: I7024446f95115b5323100bafe932cf538a32c5a2