Update AndroidX Browser Dependency in Custom Tabs Guide to 1.4.0

[jossiwolf]

Updates the version here to the latest version of AndroidX Browser which you'll want to use if you are targeting Android 12 :)

[chrome-devrel-review-bot]

Hello! This is an automated review by our custom reviewbot. It updates automatically when code or GitHub comments in this pull request are created or updated.

Requested changes

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site/en/docs/android/custom-tabs/integration-guide/index.md

• Please put a blank line before and after each section heading. Some Markdown parsers don't detect the text as a section heading unless there is blank space before and after it. Affected lines: 35
• Please change your usage of the following words:
  • Do not use master. Affected lines: 310

[jossiwolf]

I'm happy to update the doc to match the styleguide; however these lines weren't touched as part of the PR :) lmk if you want to change it, and if yes, whether in this PR or in a new one

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