[DTC] Fix NPE when finding ViewStub

Once ViewStub is inflated, it'll be remove from the view hierarchy and  findViewById() will return null.

Also fix a rebase error where observer.onHeightChanged() is called from updateViewStubTopMargin.

Bug: 1498252
Change-Id: Ieafc55cab55eda74d066d3c7bba2e06a1a91f0af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5145751
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Wenyu Fu <wenyufu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1240245}