AudioService: Allow specification of the USB safe volume.

Previously the USB safe volume was hard coded to -37.0 (see
explanation in AudioService.java). This volume is used to
determine the volume index for a safe level. The USB index can be
much lower than the similar volume index defined by
config_safe_media_volume_index, even though both may meet the EN
60950 specification (according to the test engineer).

Add a configuration property that allows the OEM to change the
USB safe volume for their device. The new property's name is
config_safe_media_volume_usb_mB. The safe volume is stored in
millibel to avoid using floating point numbers in the
configuration xml.

Bug: 73351887
Test: Default safe volume remains at -37.0dB.
      Overlay can be used to modify the safe volume.
Change-Id: I2a2ecfbc4277fddaf724f38aef7c531c71ee5c3b
(cherry picked from commit b4b2da50c942151830643aff14d394597812cb29)