AudioService: persist/restore audio device spatial audio settings

In SpatializerHelper, add support for (un)marshalling SADeviceState
(class that stores per device spatial audio settings) to/from a
String.
Restore device settings right before SpatializerHelper is
initialized. Save device settings whenever they change.
Add new test class for testing the SADeviceState marshalling
functionality, as well as compatible device list management.

Bug: 210803914
Test: atest SpatializerHelperTest
Change-Id: I94e5a32920e26dc561ede1fec4372b4a3da7f4b7