SoundTrigger test cleanup

- Remove stale stub hal test, as it is non-functional and has been
  superseded
- Create package/directory for stservice tests
- Move SoundTriggerTest to new package
- Fix SoundTriggerTest to prevent passing null to APIs which no longer
  accept null
- Split ConversionUtilTest into st service and middleware conversion
  tests

Fixes: 275434578
Test: atest FrameworkVoiceInteractionTests
Change-Id: If523b268d9defe266d998232041bcb0728554ef2