design(help): follow material guidelines

* 24dp margins around dialog
* 16dp between title and content

Author: david-allison

AnkiDroid/src/main/res/layout/dialog_help.xml

@@ -2,4 +2,7 @@
 <androidx.fragment.app.FragmentContainerView xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/fragment_container"
     android:layout_width="match_parent"
-    android:layout_height="wrap_content" />
+    android:layout_height="wrap_content"
+    android:paddingTop="8dp"
+    android:paddingBottom="24dp"
+    android:paddingHorizontal="8dp" />