audioflinger: Add session effects on spatializer mixer

Fix several issues with Spatializer mixer implementation:
- Session effects cannot be added on tracks attached to a spatializer effect.
- Post processing effects are not applied to non spatialized tracks.
- Spatializer effect is not guarantied to be inserted in first position in the
output stage session

Bug: 204742569
Test: atest AudioEffectTest
Test: atest SpatializerTest
Change-Id: I8c3185e63401eef3a5a216dc418764e7c54ab5ab