Spatial audio: read prop for spatializer init, persist feature state

Use property ro.audio.spatializer_enabled to optimize the
AudioService initialization of Spatializer.

Persist the state of the spatial audo feature whenever its state
changes. Read it at start or whenever AudioService needs to
re-initialize the feature (after audio_server restart)

Bug: 188502620
Test: adb shell dumpsys audio | grep -A 4 Spatial
Change-Id: I667ff7f0247396aad698765455e400337d9c6dcb