Do headset filter in center+side audio space

- Remove the slow integer division in the headset filter code, opting
  for more standard center+side channel calculation.
- Lay the groundwork for applying FIR-based directional emphasis
  filters. (No such filters are yet applied, however.)
- Reduce the delay line length in Haas effect to reduce audible
  ringing in the "Stage" mode.

Change-Id: If521af4c3a501be4f496de17ce179281fbd5b568