Port external helper app prompting before opening to Fenix

Trac:
Parent Ticket: legacy/trac#33661 (moved)

added component::applications/tor browser in Legacy / Trac gitlab-tb-fenix in Legacy / Trac owner::tbb-team in Legacy / Trac parent::33661 in Legacy / Trac priority::medium in Legacy / Trac severity::normal in Legacy / Trac sponsor::58 in Legacy / Trac status::new in Legacy / Trac type::task in Legacy / Trac labels

Trac:
Keywords: N/A deleted, gitlab-tb-fenix added

mentioned in issue legacy/trac#33661 (moved)

moved from legacy/trac#34378 (moved)

added Task label and removed 1 deleted label

removed 1 deleted label

mentioned in issue #34428 (moved)

mentioned in issue #34450 (closed)

added 1 deleted label

changed due date to August 29, 2020

mentioned in issue android-components#40004 (closed)

From android-components#40004 (closed). Some parts will have to be done in Fenix and some in android-components, but adding these here for now.

components/feature/app-links/src/main/java/mozilla/components/feature/app/links/AppLinksUseCases.kt
- appLinksUseCases.openAppLink used in BrowserToolbarMenuController.kt (Fenix), AppLinksFeature.kt, AppLinksInterceptor.kt and ContextMenuCandidate.kt. The only one which shows a prompt is in AppLinksFeature.kt. We should probably show a prompt for all of them.
components/feature/contextmenu/src/main/java/mozilla/components/feature/contextmenu/ContextMenuCandidate.kt
- In createShareLinkCandidate. It's in the defaultCandidates list, so it may be present in Fenix contextual menu. I'm not sure what to do with this ones (the "Sharing" ones, in general). Should we also show a prompt right before launching the app?
components/feature/downloads/src/main/java/mozilla/components/feature/downloads/AbstractFetchDownloadService.kt
- In openFile, we need to prompt.
components/support/ktx/src/main/java/mozilla/components/support/ktx/android/content/Context.kt
- In #34177 (closed) I assumed these (call intent, add contact) should not be a problem, but we probably need to double check. This is the list:
  - Context.call
  - Context.addContact
  - Context.email
  - Context.share
- These are used in several places (e.g. in HomeActivity.kt, passed to DefaultSelectionActionDelegate (to handle actions when text is selected). The only one that is not currently used is Context.addContact, since createAddContactCandidate in ContextMenuCandidates.kt is not in the default candidates, nor added in fenix BrowserFragment.kt. Not sure about these, should we also show a prompt before launching (e.g. currently in fennec-based Tor Browser we don't do it for sharing).