style: use rounded menu corners

Change-Id: Ic58ffabc6bded2f6fc61cc1f5e3e9ccc3e989515

jami-android/app/src/main/res/drawable/custom_popup_background.xml

+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <solid android:color="?attr/colorSurface"/>
+    <corners
+        android:radius="16dp"/>
+    <padding
+        android:bottom="8dp"
+        android:top="8dp"/>
+</shape>
\ No newline at end of file

jami-android/app/src/main/res/values/styles.xml

@@ -35,6 +35,8 @@
         <item name="extendedFloatingActionButtonStyle">@style/Widget.AppTheme.MainActionButton</item>
 
         <item name="preferenceTheme">@style/JamiPreferenceThemeOverlay</item>
+
+        <item name="popupMenuBackground">@drawable/custom_popup_background</item>
     </style>
 
     <style name="JamiPreferenceThemeOverlay" parent="@style/PreferenceThemeOverlay">