post_proc: fix inconsistency in virtualizer state from set_strength call

- Virtualizer is getting applied just by setting strength even though
  it was kept disabled by client.
- Enable/disable from strength context is not able to track state set by
  client, since internal state may not be same as one set by client.
- Add a separate variable to track state set by client which would help
  avoid going into any such inconsistent state.

Change-Id: I4985b2d12dc0ee3f62089423e3a71d530f66027c