Bump minSdk to 28

This change is mostly to reduce the number of APIs we need to run tests
for (from 11 APIs to 6). API 27 was released 5 years ago. Also, as of
2022-09-01, 95.1% of our monthly active users are on API 28 or higher.

uhabits-android/src/main/res/drawable-v21/widget_button_background.xml

@@ -1,39 +0,0 @@
-<!--
-  ~ Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
-  ~
-  ~ This file is part of Loop Habit Tracker.
-  ~
-  ~ Loop Habit Tracker is free software: you can redistribute it and/or modify
-  ~ it under the terms of the GNU General Public License as published by the
-  ~ Free Software Foundation, either version 3 of the License, or (at your
-  ~ option) any later version.
-  ~
-  ~ Loop Habit Tracker is distributed in the hope that it will be useful, but
-  ~ WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
-  ~ or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
-  ~ more details.
-  ~
-  ~ You should have received a copy of the GNU General Public License along
-  ~ with this program. If not, see <http://www.gnu.org/licenses/>.
-  -->
-
-<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item
-        android:top="1dp"
-        android:left="1dp"
-        android:bottom="3dp"
-        android:right="3dp">
-        <ripple
-            android:color="#60ffffff">
-
-            <item android:id="@android:id/mask">
-                <shape android:shape="rectangle">
-                    <corners android:radius="5dp"/>
-                    <solid android:color="?android:colorPrimary"/>
-                </shape>
-                <color android:color="@color/white"/>
-            </item>
-        </ripple>
-    </item>
-
-</layer-list>

uhabits-android/src/main/res/drawable/widget_button_background.xml

@@ -17,18 +17,23 @@
   ~ with this program. If not, see <http://www.gnu.org/licenses/>.
   -->
 
-<selector xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:state_enabled="true" android:state_pressed="true">
-        <layer-list>
-            <item android:bottom="3dp"
-                  android:left="1dp"
-                  android:right="3dp"
-                  android:top="1dp">
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item
+        android:top="1dp"
+        android:left="1dp"
+        android:bottom="3dp"
+        android:right="3dp">
+        <ripple
+            android:color="#60ffffff">
+
+            <item android:id="@android:id/mask">
                 <shape android:shape="rectangle">
                     <corners android:radius="5dp"/>
-                    <solid android:color="#30ffffff"/>
+                    <solid android:color="?android:colorPrimary"/>
                 </shape>
+                <color android:color="@color/white"/>
             </item>
-        </layer-list>
+        </ripple>
     </item>
-</selector>
+
+</layer-list>

uhabits-android/src/main/res/values-v21/styles_list_habits.xml

@@ -1,23 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<!--
-  ~ Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
-  ~
-  ~ This file is part of Loop Habit Tracker.
-  ~
-  ~ Loop Habit Tracker is free software: you can redistribute it and/or modify
-  ~ it under the terms of the GNU General Public License as published by the
-  ~ Free Software Foundation, either version 3 of the License, or (at your
-  ~ option) any later version.
-  ~
-  ~ Loop Habit Tracker is distributed in the hope that it will be useful, but
-  ~ WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
-  ~ or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
-  ~ more details.
-  ~
-  ~ You should have received a copy of the GNU General Public License along
-  ~ with this program. If not, see <http://www.gnu.org/licenses/>.
-  -->
-
-<resources>
-
-</resources>