Cordova W3C Battery Alignment - levelchange event

JIRA parent issue: https://issues.apache.org/jira/browse/CB-6065
Cordova documentation: https://github.com/apache/cordova-plugin-battery-status/blob/master/doc/index.md
w3c spec: http://www.w3.org/TR/battery-status

Gap analysis document can be found here: https://docs.google.com/document/d/1mAcjwf39IcIqpIuS-7SzlDxTgk4OebCHae6yfOwnStE/edit?usp=sharing

levelchange event. When the battery level is updated, the user agent mustqueue a task which sets the level attribute's value and fires a simple eventnamed levelchange at the BatteryManager object.
event handler: onlevelchange
Example:
// We get the initial value when the promise resolves ...
navigator.getBattery().then(function(battery) {
console.log(battery.level);
// ... and any subsequent updates.
battery.onlevelchange = function()

{ console.log(this.level); };
});
Example 2 (Alternative):
navigator.getBattery().then(function(battery) {
console.log(battery.level);
battery.addEventListener('levelchange', function() { console.log(this.level); }

);
});