Add basic files selector

This required to add a platform channel with a method to get the
toplevel storage devices. The app will need to request the
WRITE_EXTERNAL_STORAGE permission later. Also the dependency on "path"
was introduced.

android/app/src/main/AndroidManifest.xml

@@ -1,6 +1,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="de.johrpan.musicus">
 
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
+
     <application
         android:name="io.flutter.app.FlutterApplication"
         android:label="Musicus"

android/app/src/main/kotlin/de/johrpan/musicus/MainActivity.kt

@@ -1,12 +1,39 @@
 package de.johrpan.musicus
 
 import androidx.annotation.NonNull;
+import androidx.core.content.ContextCompat
 import io.flutter.embedding.android.FlutterActivity
 import io.flutter.embedding.engine.FlutterEngine
+import io.flutter.plugin.common.MethodChannel
 import io.flutter.plugins.GeneratedPluginRegistrant
 
-class MainActivity: FlutterActivity() {
+class MainActivity : FlutterActivity() {
+    private val CHANNEL = "de.johrpan.musicus/platform"
+
     override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
-        GeneratedPluginRegistrant.registerWith(flutterEngine);
+        GeneratedPluginRegistrant.registerWith(flutterEngine)
+
+        MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler { call, result ->
+            if (call.method == "getStorageRoots") {
+                result.success(getStorageRoots())
+            } else {
+                result.notImplemented()
+            }
+        }
+    }
+
+    private fun getStorageRoots(): ArrayList<String> {
+        val result = ArrayList<String>()
+
+        ContextCompat.getExternalFilesDirs(this, null).forEach {
+            val path = it.absolutePath;
+            val index = path.lastIndexOf("/Android/data/")
+
+            if (index > 0) {
+                result.add(path.substring(0, index))
+            }
+        }
+
+        return result
     }
 }