Fix regression on voice call default volume

This CL fixes regression introduced by aosp/586122.
The fallback of default volume uses a temporary max volume which may
not be valid. (-1).
This CL uses the up to date AudioSystem.DEFAULT_STREAM_VOLUME instead.
This happens only if no "ro.config.vc_call_vol_default" property defined.

Test: Ensure "ro.config.vc_call_vol_default" is not defined &&
    dumpsys audio && ensure VOICE CALL default volume (aka for device 40000000)
    is not zero.

Signed-off-by: François Gaffie <francois.gaffie@renault.com>
Change-Id: I8937e54a61df196da8c2578f0a581b63dea34caa