Allow mipmap's as icon

It would be nice to use `mipmap`'s as notification icon like the [new launcher icon](https://developer.android.com/studio/write/image-asset-studio)

for example add this snippet [here](https://github.com/EddyVerbruggen/nativescript-feedback/blob/master/src/feedback.android.ts#L132):
```
let defType = 'drawable';
let segments = resourcename.split('/');
if (segments.length > 1) {
defType = segments[0];
resourcename= segments[1];
}
```

so as `icon` property could be set: `mipmap/icon` or `icon` (to get it from `drawable`)

Start in src/feedback.android.ts around line 132, where the notification icon resource is resolved. Trace the existing icon handling, then verify that both mipmap/icon and an unqualified icon resolve correctly, with the latter remaining in drawable; no test file is mentioned in the issue.

Written by the indexing model from the issue text.