Updates to the sound trigger test app

Moved the logic for talking to the system service into a service in the
app - this now lets you close the activity and still be able to receive
events. Additionally, we get CLI support using commands with the intent
com.android.intent.action.MANAGE_SOUND_TRIGGER.

Bug: 29073629

Change-Id: Ie904b73b4414f2c9fded013aeb5e6c6c3a67f5d3