Rename feature flag, to allow for separate flags for each password import promo

[CDRussell]

Task/Issue URL: https://app.asana.com/1/137249556945/project/608920331025315/task/1210639532381271?focus=true

Description

Renames feature flag for password import promo in password management screen (will introduce another for password import promo in browser)

Steps to test this PR

• qa optional

[CDRussell]

• Create dialog inside autofill-impl instead of BTF being responsible for it #6368
• Add in-browser prompt to import Google Passwords #6290
• 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 👈 (View in Graphite)
• 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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[CDRussell]

Will need to tweak ImportInPasswordsVisibility in higher PR (when there is the second promo) but just renaming that existing flag for now

[CDRussell]

Merge activity

• Jul 2, 9:09 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 2, 9:59 AM UTC: Graphite rebased this pull request as part of a merge.
• Jul 2, 10:13 AM UTC: @CDRussell merged this pull request with Graphite.