SpatializerHelper: fix setting desired head tracking mode

When the desired head tracking mode changes, do not compare against
current actual mode, as transient mode could be equal to the
desired mode. Instead always set the desired mode when the value
changes.
Also move head tracker / screen sensor init to dedicated methods
for code readability.

Bug: 225220771
Test: 5.1 playback with head tracker
Change-Id: I1147f6fa5c74a017c32edfe2ff19a578d6b38470