Refactor capacitor plugins, clean up and organize android plugin classes

plugins/capacitor/index.js

@@ -0,0 +1,13 @@
+import Vue from 'vue'
+import { AbsAudioPlayer } from './AbsAudioPlayer'
+import { AbsDownloader } from './AbsDownloader'
+import { AbsFileSystem } from './AbsFileSystem'
+import { Capacitor } from '@capacitor/core'
+
+Vue.prototype.$platform = Capacitor.getPlatform()
+
+export {
+  AbsAudioPlayer,
+  AbsDownloader,
+  AbsFileSystem
+}