This includes UI in private tabs.

Squashed commits:
[180e515e8] Connect to Session Observer and thence to AppLinksFeature.
[77922dcc0] browser-engine-system tests passing
[045717868] lint etc
[6f7b44cad] Thread another Boolean through EngineObserver -> Session onLoadRequest
[18693cad0] Start testing the feature.
[5e0a93556] Fixup tests to use shadow package manager, and catch some new bugs.
[3a5976dde] Split use cases up for menu and intercepting cases.
[f7eae96dd] Address reviewer comments re: dead code and docs
[b15ffb88c] Rename triggeredByUserInteraction to loadRequestTriggeredByUserInteraction
[edc49b573] Move session observer into member of feature
[91ed1f5ac] Add changelog entry
[098dc5067] Add boolean flag to prevent interception of links
[f92731a44] Add load request/trigger by user interaction to session.observer
[46385577d] Move opening to another use case, with test
[56da7e7c1] Change into using feature
[643c9f453] Remove duplicate copyright
[6b3f80bbc] Add testing of get and a fragment for open use cases
[669c84567] Add AppLinksRedirectConfig and supporting use cases.

There is a url redirect creation use case, a load URL use case and an interceptor.
[72896a567] AppLinksUseCases detects if a URL can be opened with another non-browser app
[939eb62fa] Initial commit of empty feature-app-links