[Bug] No indication that WebExtensions don't run in private browsing mode

[Rob--W]

Related: #6123 (introduced the ability to turn off add-ons in private browsing mode)
Related: #10140 (user confusion)
Related: #10244 (indicator in addon list that shows whether access to private browsing is allowed)

Steps to reproduce

Install an addon, e.g. uBlock Origin.

Expected behavior

The user should be notified that the add-on does not have access to private browsing mode, with an opt-in. On Firefox desktop, there is a post-install doorhanger where the user can tick a checkbox to allow access: https://support.mozilla.org/en-US/kb/extensions-private-browsing#w_granting-permission-on-installation

Actual behavior

There is no indication whatsoever that the newly installed add-on isn't able to access content that runs in private browsing mode.

Device information

• Android device: ?
• Fenix version: Nightly 200503 06:01 (build #21240608)

┆Issue is synchronized with this Jira Task

[brampitoyo]

@eliserichards One of my initial thinking about add-ons and private mode was #6123 (comment):

I think that many users’ expectation is for add-ons to work everywhere by default [ie. enabled in private mode], but also for those add-ons to not leak data while browsing privately.

@csadilek’s response was #6123 (comment):

I think defaulting to "false" is the right thing to do here, so users can opt-in, as desired. Having a list of recommended and tested extensions protects us from this somewhat, but it still could become a problem later. So, I'd vote for being conservative and use the same default as Desktop.

At the end of the add-on install process, we’ve shown a non-blocking snackbar. To solve this problem, this snackbar will need to be changed into a blocking modal bottom sheet. This way, users will be given a chance to permit add-ons to run in private mode.

Stay tuned for the design of this modal bottom sheet!

[brampitoyo]

@Amejia481 modal bottom sheet mockup attached below.

[AndiAJ]

Hi, verified as fixed on the latest Nightly Build 200515 from 5/15 using the following devices:
• Google Pixel 3a (Android 10)
• Huawei Mate 20 Lite (Android 9)
• OnePlus A3 (Android 6.0.1)

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