audio service: add config option for fixed volume

audio service: add config option for fixed volume

Add a boolean configuration option config_useFixedVolume indicating if
stream volumes or master volume can be modified.
If the option is true, the AudioManager volume and mute APIs will be no ops and the
volumes will be maxed out.
To be consistent:
- the ringer mode is forced to normal and cannot be modified
- volume panel is never displayed
- volume settings are not available
- ringer mode global action is not displayed.

The default for this option if false.

This is useful for a class of devices intended for connection to a digital
audio output only, where the volume is directly controlled on the audio sink.

Bug 8161458

Change-Id: I2571d5ee79952ef0914d8fd1985816467a80adcd