Only selected VoiceInteractionService can call methods

- Extract current code for checking caller into a private method
- Replace occurrences of that code with a call to the private method
- Add method call to setTranscription/clearTranscription/setVoiceState

Test: Verified Milford can't call setTranscription/clearTranscription
when it is no longer the active service.
Bug: b/123412646

Change-Id: I2c428c6c65b62f6a83264286df4f44fb5d1c249e