audio policy: support platforms with no audio devices

Remove requirement to declare an AUDIO_DEVICE_OUT_SPEAKER device
for platforms without audio output.

By convention:
- platforms without audio output should declare a single output
device of type AUDIO_DEVICE_OUT_STUB also declared as default
output device
- platforms without audio input should declare a single input
device of type AUDIO_DEVICE_IN_STUB

Platforms with no audio at all can use stub audio policy configuration
file and audio HAL with the following instructions in device.mk file:

USE_XML_AUDIO_POLICY_CONF := 1
PRODUCT_PACKAGES += \
    audio.stub.default
PRODUCT_COPY_FILES += \
    frameworks/av/services/audiopolicy/config/audio_policy_configuration_stub.xml:system/etc/audio_policy_configuration.xml

Bug: 25075342
Change-Id: Id700978559427edd3c7cc38d98f2fd52928367ed
diff --git a/services/audiopolicy/AudioPolicyInterface.h b/services/audiopolicy/AudioPolicyInterface.h
index b1347f4..a215b95 100644
--- a/services/audiopolicy/AudioPolicyInterface.h
+++ b/services/audiopolicy/AudioPolicyInterface.h
@@ -158,14 +158,14 @@
                                       int indexMax) = 0;
 
     // sets the new stream volume at a level corresponding to the supplied index for the
-    // supplied device. By convention, specifying AUDIO_DEVICE_OUT_DEFAULT means
+    // supplied device. By convention, specifying AUDIO_DEVICE_OUT_DEFAULT_FOR_VOLUME means
     // setting volume for all devices
     virtual status_t setStreamVolumeIndex(audio_stream_type_t stream,
                                           int index,
                                           audio_devices_t device) = 0;
 
     // retrieve current volume index for the specified stream and the
-    // specified device. By convention, specifying AUDIO_DEVICE_OUT_DEFAULT means
+    // specified device. By convention, specifying AUDIO_DEVICE_OUT_DEFAULT_FOR_VOLUME means
     // querying the volume of the active device.
     virtual status_t getStreamVolumeIndex(audio_stream_type_t stream,
                                           int *index,