audiopolicy: Avoid logspam about engine config parsing error

Skip calling into libxml2 parsing when the engine config
file does not exist (this is allowed). This removes a misleading
error message from libxml2 in the syslog.

Bug: 181269159
Test: adb shell logcat | grep libxml2
      during phone boot
Change-Id: I9159a458ce68146271c3b22ea452194c4cabc79f
1 file changed