Use check box as complete icon

app/src/main/java/com/todoroo/astrid/activity/TaskEditFragment.java

@@ -177,13 +177,13 @@ public final class TaskEditFragment extends InjectingFragment
       binding.fab.setVisibility(View.INVISIBLE);
     } else if (completed) {
       title.setPaintFlags(title.getPaintFlags() | Paint.STRIKE_THRU_TEXT_FLAG);
-      binding.fab.setImageResource(R.drawable.ic_undo_24px);
+      binding.fab.setImageResource(R.drawable.ic_outline_check_box_outline_blank_24px);
     }
     binding.fab.setOnClickListener(v -> {
       if (completed) {
         completed = false;
         title.setPaintFlags(title.getPaintFlags() & ~Paint.STRIKE_THRU_TEXT_FLAG);
-        binding.fab.setImageResource(R.drawable.ic_check_black_24dp);
+        binding.fab.setImageResource(R.drawable.ic_outline_check_box_24px);
       } else {
         completed = true;
         save();

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

@@ -167,10 +167,8 @@
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_margin="@dimen/keyline_first"
-      android:src="@drawable/ic_check_black_24dp"
+      android:src="@drawable/ic_outline_check_box_24px"
-      android:tint="?attr/colorOnSecondary"
-      android:background="?attr/colorSecondary"
       app:layout_anchor="@id/appbarlayout"
-      app:layout_anchorGravity="bottom|right|end" />
+      app:layout_anchorGravity="bottom|end" />
 
 </androidx.coordinatorlayout.widget.CoordinatorLayout>