AudioService: reduce EU volume warning thresdhold for USB headsets

Take into account additional gain due to simultaneous application of
EQ and bass boost by substracting 7dB from current warning message threshold.

Bug: 65225835
Test: check warning displayed when playing over USB headset after boot when
no SIM is present

Change-Id: I1bfcbf571d72122f4df6ff5e40be227c42d555e3