Use scoped app storage on Android (#11466)

From November 2021, the Play Store will no longer be accepting
apps which use the deprecated getExternalStorageDirectory() API.

Therefore, this commit replaces uses of deprecated API with the new
scoped API (`getExternalFilesDir()` and `getExternalCacheDir()`).
It also provides a temporary migration to move user data from the
shared external directory to new storage.

Fixes #2097,  #11417 and #11118

android/app/src/main/res/layout/activity_main.xml

@@ -1,4 +1,5 @@
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+	xmlns:tools="http://schemas.android.com/tools"
 	android:id="@+id/activity_main"
 	android:layout_width="match_parent"
 	android:layout_height="match_parent"
@@ -14,7 +15,8 @@
 		android:layout_marginRight="90dp"
 		android:indeterminate="false"
 		android:max="100"
-		android:visibility="gone" />
+		android:visibility="gone"
+		tools:visibility="visible" />
 
 	<TextView
 		android:id="@+id/textView"
@@ -25,6 +27,7 @@
 		android:background="@android:color/transparent"
 		android:text="@string/loading"
 		android:textColor="#FEFEFE"
-		android:visibility="gone" />
+		android:visibility="gone"
+		tools:visibility="visible" />
 
 </RelativeLayout>

android/app/src/main/res/values/strings.xml

@@ -3,9 +3,11 @@
 
 	<string name="label">Minetest</string>
 	<string name="loading">Loading&#8230;</string>
+	<string name="migrating">Migrating save data from old install&#8230; (this may take a while)</string>
 	<string name="not_granted">Required permission wasn\'t granted, Minetest can\'t run without it</string>
 	<string name="notification_title">Loading Minetest</string>
 	<string name="notification_description">Less than 1 minute&#8230;</string>
 	<string name="ime_dialog_done">Done</string>
+	<string name="no_external_storage">External storage isn\'t available. If you use an SDCard, please reinsert it. Otherwise, try restarting your phone or contacting the Minetest developers</string>
 
 </resources>