For #5334: added private custom tab processor

[severinrudie]

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Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

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[codecov-io]

Codecov Report

Merging #6845 into master will increase coverage by 0.11%.
The diff coverage is 64.1%.

@@             Coverage Diff              @@
##             master    #6845      +/-   ##
============================================
+ Coverage     17.34%   17.46%   +0.11%     
- Complexity      392      394       +2     
============================================
  Files           281      281              
  Lines         11228    11237       +9     
  Branches       1547     1551       +4     
============================================
+ Hits           1948     1962      +14     
+ Misses         9127     9120       -7     
- Partials        153      155       +2

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/org/mozilla/fenix/theme/ThemeManager.kt	0% <ø> (ø)	0 <0> (ø)	⬇️
...lla/fenix/customtabs/ExternalAppBrowserActivity.kt	41.66% <ø> (+3.2%)	4 <0> (ø)	⬇️
.../fenix/browser/browsingmode/BrowsingModeManager.kt	100% <ø> (+33.33%)	0 <0> (ø)	⬇️
.../mozilla/fenix/customtabs/CustomTabsIntegration.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...a/org/mozilla/fenix/components/IntentProcessors.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...lla/fenix/customtabs/ExternalAppBrowserFragment.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...n/java/org/mozilla/fenix/IntentReceiverActivity.kt	71.11% <89.28%> (+12.28%)	5 <0> (-1)	⬇️
...la/fenix/shortcut/NewTabShortcutIntentProcessor.kt	75% <0%> (+16.66%)	5% <0%> (+3%)	⬆️

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