Delay setting scroll position until Recycler is ready for it

[CDRussell]

Task/Issue URL:

• Internal: https://app.asana.com/0/414730916066338/1200504463069262/f
• Fixes Opening a background tab and going back to tabs makes tabs window to scroll to top #1273

Description:
Ensures the RecyclerView in the tab switcher doesn't have its scroll position set until it's ready to receive that

Steps to test this PR:

On develop

1. Open up 12 tabs (hint, wikipedia is a good site to use to find lots of links to open in background)
2. Switch to the tab in position 7 or 8
3. Open tab switcher again; you might see the bug whereby it doesn't scroll (repeat a few times if you don't see it immediately)
4. Install this branch
5. Repeat the experiment; you should see it set scroll position correctly
6. Repeat for other elements in the list (first, last etc...) and confirm all work as expected

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Internal references:

Software Engineering Expectations
Technical Design Template

[cmonfortep]

LGTM 👍