AudioService: SA feature enabled doesn't rely on global setting

 Unlike Android S, there is no more global toggle for spatial
audio, whether the feature is enabled has become contextual
based on the current routing.
 The fix consists in ignoring the persisted value for the
feature (Settings.Secure.SPATIAL_AUDIO_ENABLED) and only
rely on the effect availability for enabling the feature or not.

Bug: 227774516
Test: adb shell dumpsys audio | grep -A 20 "Spatial audio"
Change-Id: I4fb7b65b3bdc5cbefefcbc027e385917e2586e0f