Merge/release 17.6 into develop

[AliSoftware]

⚠️ This ideally requires #14927 to be merged into release/17.6 first – but if it gets approved we can merge this all at once because that other PR is already included in this one (see below).

This PR is intended to merge release/17.6 into develop after doing 17.6-rc-4, which includes:

• Revert app icons background update #14882
• Jetpack App: Update infrastructure to support Jetpack app #14613
• Integrate gutenberg-mobile release 1.55.2 #14892
• [Tooling] Fix version names and codes per variant #14927

Note that:

• This uses an internediate merge/release-17.6-into-develop branch to fix conflict – which was mainly on the ext.gutenbergMobileVersion value that is already on v1.56.0-alpha4 in develop while it's v1.55.2 in release branch, so kept the one from develop during conflict resolution.
• But that branch in fact got cut from fix-version-names-and-codes-per-variant and not release/17.6, because that's the branch that was used to do the release (and where the 17.6-rc-4 tag landed), and we're waiting for [Tooling] Fix version names and codes per variant #14927 to be merged into release/17.6 first.

Ideally once #14927 is merged into release/17.6, one could rebase merge/release-17.6-into-develop on top of release/17.6 for consistency before merging this PR… but in practice we want this to be merged ASAP to be ready for the final release that must happen sooner than usual, and since I'm at my EOD now, it's not worth waiting for me to rebase only on my tomorrow morning before merging, better get this land ASAP. It should lead to the same end result if we merge this PR as-is (including changes from both PRs at once) anyway.

                                    17.6-rc-4     ┌───────────────────────────────────────┐
                                  ○──○──●─────◇   │fix-version-names-and-codes-per-variant│
                                 ╱             ╲  └───────────────────────────────────────┘
                                ╱         ╲     ╲
                               ╱                 ╲
                              ╱             ╲     ╲
╔══════════════╗             ╱            #14927   ╲
║ release/17.6 ║    ○ ─ ─○──○─────────────────◌ ─ ─ ╲ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─▶
╚══════════════╝   ╱                                 ╲
                  ╱                                   ╲     fix
                 ╱                                     ╲  conflict    ┌───────────────────────────────┐
                ╱                                       ○━━━━━━━━━━━○ │merge/release-17.6-into-develop│
               ╱                                       ╱              └───────────────────────────────┘
              ╱                                       ╱               ╲
 ╔═══════╗   ╱                                       ╱                  this PR
 ║develop║──○──────○───○───○───────○────────────────○───────────────────◌────▶
 ╚═══════╝

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[AliSoftware]

Points to the new google-services.json with the correct signing fingerprint for Jetpack

[AliSoftware]

Pointing to a pending commit for now waiting for wordpress-mobile/release-toolkit#283 to be merged and release.

IMHO it's ok to merge that PR in develop even in that temp state, and update to point to 1.3.1 later once toolkit 1.3.1 gets released (if the release-toolkit new version takes time to land, that is). Better sync with develop sooner than later and regularize the toolkit version on second stage once ready, than wait for everything, because we're kind of in a rush with the #wp-breaking-02 requiring expedite final release of 17.6 sooner than usual.

[AliSoftware]

This comes from #14927: versioning is now handled by the version.properties file – so that it's easier to support app-and-flavor-specific versioning and disinguish Jetpack, WordPress and WordPress Alpha. See below for how the versionName and versionCode are how overriden per variant and not per flavor.