Add check to cache before bundle and full path support.

To try the cache, you can add this in EjectaRenderer.java:
```Java
Utils.copyDatFiles(ctx, dataBundle + "/cache/", "www");
```

In index.js, you can try this:
```JavaScript
img.src = '/data/data/com.impactjs.ejecta.sample/cache/bg.png';
```
```JavaScript
img.src = 'file:///data/data/com.impactjs.ejecta.sample/cache/bg.png';
```
```JavaScript
img.src = 'bg.png';
```