feat(android): support custom local notification icon

[nikosdouvlis]

Support for android custom local notification icon.

Added a smallIcon property to LocalNotificatoin. Can accept values of both res://icon_name and icon_name types (to stay consistent with the attachments' url prop). Will only look inside the res/drawables directory since the mipmap one is supposed to be used only for launcher icons.

If no custom icon is set or if the custom one cannot be found, the standard android info icon will be used instead.

If this gets merged, the docs need to be updated to make clear that the custom icon needs to follow Android's guidelines (see the top answer here: https://stackoverflow.com/questions/45318614/why-is-my-smallicon-for-notifications-always-greyed-out) or else the it won't be displayed.

Closes #1752
Closes #1510

[jcesarmobile]

Added a comment, but not sure if the icon should be provided every time as part of the notification payload or if it's better to make a plugin preference to be added in the plugins.LocalNotifications or android sections of capacitor.config.json, or even in the AndroidManifest.xml as the push icon, that way configuring it once is enough. Or maybe both approaches but if it's passed as part of the payload make it take precedence?

[nikosdouvlis]

My initial thought was to have an API similar to cordova-plugin-local-notifications. Since it was the most popular cordova local notifications plugin I guess most of the teams migrating from cordova to capacitor are already familiar with.

But I guess you're right, most apps (like ours) won't need to configure the icon for each notification individually - so a global config makes sense.

So I pushed a new commit, which allows configuring the smallIcon in capacitor.config.json like:

  "plugins": {
    "LocalNotifications": {
      "smallIcon": "ic_stat_icon_config_sample"
    }
  }

but also keeps the previous functionality - allowing to override the default icon per notification.

[jcesarmobile]

You've put the icons on capacitor project instead of in the example project.
I've moved them to the proper place so those sample icons don't get distributed on every capacitor project.

Other than that, looks good to me, will merge once tests finish.

Thanks!