audio policy: do not route accessibility prompts to compressed output

When a digital output (HDMI or S/PDIF) is configured with
a non linear PCM audio format it is likely that the audio HAL
or HW will not be able to mix other sounds with the main
media stream being played.

In this case, do not condider HDMI or S/PDIF as a valid route for
accessibilty strategy so that accessibility prompts are always audible.

Bug: 18067208.
Change-Id: If20603a1dc1dc470f206f841308bed2b87daf21c