Not all devices will have power+volume chord enabled. Fix current wrong behaviour by falling back to mute chord behaviour only if it was set as the default option in device overlay.
Bug: 187814162
Test: make RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.gestures.*"
Change-Id: I15c0a71d25738b3b01a20bf22c790d9dcba5d6fd