Add in-browser prompt to import Google Passwords

[CDRussell]

Task/Issue URL: https://app.asana.com/1/137249556945/project/608920331025315/task/1210270953487596?focus=true

Description

Steps to test this PR

Feature 1

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UI changes

Before	After
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• To see the specific tasks where the Asana app for GitHub is being used, see below:
  • https://app.asana.com/0/0/1210729428247645

[CDRussell]

• Create dialog inside autofill-impl instead of BTF being responsible for it #6368
• Add in-browser prompt to import Google Passwords #6290 👈 (View in Graphite)
• Force line break in import promo title #6317
• Add dev setting for clearing previous google import flag #6311
• Add source to import flow pixels #6310
• Update promo CTA's copy and remove its icon #6307
• Rename feature flag, to allow for separate flags for each password import promo #6293
• Add icon to the import promo's CTA button #6282
• Change requirement for marking a previous password import as having happened #6281
• Don't show preimport dialog when launching from password import promo #6278
• Show import promo in autofill management #6266
• develop

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