Added M3 theme to intro pages

[ArnyminerZ]

(See #691)

Purpose

Right now our M3 theme is not being used for intro pages.

Short description

• Added AppTheme call in PageFragment.
• Added a TODO to remember removing the old M2 theme once all migrations are complete for intro.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

Depends on #739
Depends on #740
Depends on #741
Depends on #757
Depends on #759

[rfc2822]

We shouldn't have M2Theme and AppTheme at the same time. We can merge this when all pages are rewritten and then only need AppTheme. I have set appropriate dependencies.

[sunkup]

We shouldn't have M2Theme and AppTheme at the same time. We can merge this when all pages are rewritten and then only need AppTheme. I have set appropriate dependencies.

Okay, but why?

[rfc2822]

I don't want to need multiple PRs for the same file for the rewrite except it's really required without alternative. But I'd prefer that no files are "in-between", but either "old" or "rewritten/finished".

[github-actions]

This PR/issue depends on:

• Rewrite WelcomePage to M3 #739
• Rewrite OpenSourcePage to M3 #740
• Rewrite BatteryOptimizationsPage to M3 #741
• Rewrite PermissionsIntroPage to M3 → PermissionsActivity #757
• Rewrite TasksIntroPage to M3 → TasksActivity #759