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