Use browserstore instead of browsing mode manager #414 (firefox-autoland@autoland)

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Branches use_browserstore_instead_of_browsing_mode_manager -> autoland
Repo firefox-autoland@autoland (git) (git://github.com/Mugurell/firefox.git)
Author Mugurell
State open
Commit Title

Use browserstore instead of browsing mode manager

Commit Body

This is an alternative to reading the browser mode from `AppState` - https://github.com/mozilla-firefox/firefox/pull/409 The change that this PR brings: Each Fenix activity takes its private or normal mode from the tab it shows, instead of sharing one app-wide mode. A custom tab and the regular tabs activity can sit side by side in split screen, each keeping its own theme, lock, prompts and new-tab privacy, without changing the other. A longer description, edgecases covered and accepted trade-offs are available [here](https://docs.google.com/document/d/1v7VWZXPHOBqM1x_XRbp1gjexyE4AeAt84UyWQh6hL-4/edit?usp=sharing).

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