Don't clear toasts when changing users

[gaearon]

While working on the session code I've noticed we have a bunch of weird setTimeouts around toasts, and that toasts work poorly when switching accounts in general. Turns out, it's because we unmount them.

In this PR, I've relaxed it so that toasts are placed outside the area that gets remounted on user change. As a result, I was able to remove the fragile setTimeout hacks. See individual commits.

Reviewing

• dcb1475?w=1: Puts legacy theme provider next to Alf provider.
• 38a02b8?w=1: Moves RootSiblingParent upwards. This is where toasts get mounted on native. I'll assume we only use it for toasts so it's fine to bleed previous users' UI on account change.
• b54e358?w=1: On web, we use this to render toasts. Make sure it also doesn't get remounted.
• 1d8ad44?w=1: We can remove the hacks now.
• 06e20b2: Our session code does emitSessionDropped in the onAgentSessionChange handler. It seems like it makes sense to show a toast whenever that happens, regardless of the reason (since it can happen out of the blue too). The native App.native.tsx component already does this by listenSessionDropped so this copy-pastes the same approach into App.web.tsx. I presume it wasn't previously enabled due to the remounting issues.
  • This also lets us remove the toast in useAccountSwitcher. It will be taken care of by the emitSessionDropped logic. In fact, this toast isn't even being used yet anyway. It was added in c020a4c#r1584118170 but not hooked up (cause initSession never throws atm), and I think we don't need it now either due to the new strategy.

Test Plan

Switch accounts on iOS and web. Observe the toast showing up. Try doing it via Settings, via quick switcher, and via the Choose Account screen.

Force session expiry on web using the trick I described in the test plan for #3838. Observe the expiry toast showing up.

[render]

Your Render PR Server URL is https://social-app-pr-3843.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-coqg18o21fec73am82v0.

[github-actions]

Old size	New size	Diff
6.86 MB	6.86 MB	70 B (0.00%)

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[mozzius]

Makes sense, and looks good

[pfrazee]

Oh nice