[Autofill Assistant] Fix animation bug when starting regular script.

When starting a regular script after a trigger script was shown, there
used to be an animation glitch while the bottom sheet resized. This was
caused by the progress bar repopulating all of its views.

This CL is a hotfix for this issue. Adding the new icons before
clearing the old ones ensures that the height of the progress bar never
changes.

Bug: b/174014637
Change-Id: I37e836a7f579cefc227feddabcbf8cb864a6bbe6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554919Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#830210}

chrome/android/features/autofill_assistant/java/src/org/chromium/chrome/browser/autofill_assistant/header/AssistantStepProgressBar.java

@@ -380,9 +380,11 @@ public class AssistantStepProgressBar {
     }
 
     public void setSteps(List<AssistantDrawable> icons) {
-        mView.removeAllViews();
-
         assert icons.size() >= 2;
+        // NOTE: instead of immediately removing the old icons, we first add the new ones. This
+        // prevents a change of the bottom sheet height, which was the cause for animation glitches
+        // (see b/174014637).
+        int oldNumChilds = mView.getChildCount();
         mNumberOfSteps = icons.size();
         mCurrentStep = -1;
 
@@ -399,6 +401,8 @@ public class AssistantStepProgressBar {
                         Locale.getDefault(), AssistantTagsForTesting.PROGRESSBAR_LINE_TAG, i));
             }
         }
+
+        mView.removeViews(0, oldNumChilds);
     }
 
     public void setVisible(boolean visible) {