[audio_core] Rectify logging levels

To prepare for enabling very-long-running tests, as well
as for the introduction of a framework for telemetry
collection, this CL makes consistent the use of logging
by audio_core (and audio_core's tests). Truly blocking
conditions are ERROR; safely-ignored conditions are INFO.
Also, for debug builds audio_core's default log level is
set to INFO, while on release builds it is set to WARNING.

Test: no functional behavior change
Change-Id: I1d8251a6aac79fc0bfb811ca8c398f2ac2004818