Auto-recenter head

This logic detects when the head has been approximately still for a
certain amount of time and triggers a head-recenter.

Documentation changes to follow.

Test: Integrated this code into the Spatial Audio Demo app and
      manually verified.
Test: Ran the included units tests.
Change-Id: I22b9e590aa1ca725d43b78fc58ade1144f2e4e52