[iOS][LU] Remove placeholder on LVF dismiss

In this CL:
- When dismissing LVF, the placeholder view should be removed to avoid a visual flicker
- For robustness, when displaying LVF check for presented view controllers and defer presentation until their dismissal if needed

Change-Id: I6eecd05dafb7119a854318c4b4ec3000f584696f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6579031
Reviewed-by: Christian Xu <christianxu@chromium.org>
Commit-Queue: Radu Nitescu <radunitescu@google.com>
Cr-Commit-Position: refs/heads/main@{#1464700}