audio policy: add ECHO_REFERENCE to virtual audio sources

When ECHO_REFERENCE audio source was added, it was omitted
from the list of virtual audio sources (sources protected by privileged
permission not accessing the mic). This prevents from capturing echo
reference at the same time as the input from mic.

Bug: 154639757
Test: test app on yukawa
Change-Id: Ib12e8c817dec7c3f4d3f3f7e7403330c88cead52