CB-10554: Implementing plugin save/restore API for Android

[riknoll]

The purpose of this PR is to handle the case where the Webview gets destroyed in the background while a capture activity is in the foreground. #51 was invalidated by some updates that were recently made to the plugin to handle overlapping permission and activity requests.

Also worth noting is that I did something different than the usual resume event API for getting pending plugin callback results. Instead of returning the native result to the javascript directly, this plugin needs to first wrap the results returned by the native code in a MediaFile object for those results to be useful. For that reason, I altered the plugin to expose two document events on Android that the user can subscribe to rather than the cordova-android generated resume event. These events directly return the wrapped result rather than the raw native one. It ends up looking like this:

function onDeviceReady() {
    // pendingcaptureresult is fired if the capture call is successful
    document.addEventListener('pendingcaptureresult', function(mediaFiles) {
        // Do something with result
    });

    // pendingcaptureerror is fired if the capture call is unsuccessful
    document.addEventListener('pendingcaptureerror', function(error) {
        // Handle error case
    });
}
document.addEventListener('deviceready', onDeviceReady);

Whereas using the resume event directly would require us to expose a helper API on Android:

function onDeviceReady() {
    document.addEventListener(resume, function(event) {
        if(event.pendingResult && event.pendingResult.pluginServiceName === "Capture") {
            if(event.pendingResult.pluginStatus === "OK") {
                var rawMediaFiles = event.pendingResult.result;
                // Here, we would need to expose some weird helper function to deal with the native result
                var mediaFiles = capture.wrapMediaFiles(mediaFiles);
                // Do something with result
            } else {
                var error = event.pendingResult.result;
                // Handle the error case
            }
        }
    });
}
document.addEventListener('deviceready', onDeviceReady);

I think this makes the API a bit cleaner and also opens the way for plugins that do much more heavy javascript work before returning callback results. This hasn't really come up before because the other two core plugins that had this issue (camera and contacts) could just pass the native result directly to the javascript.

I'd appreciate some feedback on this API and if it seems okay I might update the Android plugin guide with an example so that other plugins can use this pattern.

[riknoll]

@adamduren @nikhilkh @infil00p I'd appreciate some feedback.

[nikhilkh]

LGTM - Consider exposing these events on the capture object instead of document