Add update checker back, with updated handling. Check description.

FlavorTown will check if current app was installed from F-droid and not show the update dialog if true.
You can now skip an update.
You can now force check for an update by clicking More -> Version (even if you set to skip the update).

app/src/main/res/drawable/preference_list_divider_material.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?><!--
+  Copyright (C) 2015 The Android Open Source Project
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+       http://www.apache.org/licenses/LICENSE-2.0
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License
+  -->
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <solid android:color="#1f000000" />
+    <size
+        android:width="1dp"
+        android:height="1dp" />
+</shape>

app/src/main/res/layout/dialog_update.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:orientation="vertical"
+    android:padding="16dp">
+
+    <androidx.appcompat.widget.AppCompatTextView
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="@string/update_notice"
+        android:textAppearance="?attr/textAppearanceBody1" />
+
+    <CheckBox
+        android:id="@+id/skip_update"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="@string/skip_update_checkbox" />
+
+</LinearLayout>

app/src/main/res/layout/item_pref_divider.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<View xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="1dp"
+    android:background="@drawable/preference_list_divider_material" />

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

@@ -251,4 +251,6 @@
     <string name="logout_success">Successfully logged out!</string>
     <string name="dm_thread_info">Info</string>
     <string name="mark_as_seen">Mark as seen</string>
+    <string name="skip_update_checkbox">Do not show again until next update</string>
+    <string name="version">Version</string>
 </resources>