refactor(isOnTop): add IsOnTop.java back

mastodon/src/main/java/org/joinmastodon/android/fragments/IsOnTop.java

@@ -0,0 +1,13 @@
+package org.joinmastodon.android.fragments;
+
+import androidx.annotation.Nullable;
+import androidx.recyclerview.widget.RecyclerView;
+
+public interface IsOnTop {
+	boolean isOnTop();
+
+	default boolean isRecyclerViewOnTop(@Nullable RecyclerView list) {
+		if (list == null) return true;
+		return !list.canScrollVertically(-1);
+	}
+}