Add helper for underlining text

[NotWoods]

Adding an underline to a TextView seems to be a fairly common pattern, so lets put it in a helper method. Also adds more tests to some of the changed components.

[jhugman]

Everytime I see PR like this, I pour one out for Netbeans Jackpot.

[jhugman]

I think I'd prefer an extension property here, or at least make the method take an optional start, end or start, len arguments.

[NotWoods]

What sort of property are you thinking of? I'd prefer not to add the arguments as all the uses of UnderlineSpan just underline the entire string.

[codecov-commenter]

Codecov Report

Merging #12520 into master will increase coverage by 0.20%.
The diff coverage is 78.57%.

@@             Coverage Diff              @@
##             master   #12520      +/-   ##
============================================
+ Coverage     24.73%   24.94%   +0.20%     
- Complexity      828      830       +2     
============================================
  Files           388      389       +1     
  Lines         15668    15625      -43     
  Branches       2024     2019       -5     
============================================
+ Hits           3875     3897      +22     
+ Misses        11480    11417      -63     
+ Partials        313      311       -2     

Impacted Files	Coverage Δ	Complexity Δ
...org/mozilla/fenix/home/tips/ButtonTipViewHolder.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...g/mozilla/fenix/settings/logins/SavedLoginsView.kt	17.14% <0.00%> (+1.35%)	0.00 <0.00> (ø)
...oncontrol/viewholders/onboarding/OnboardingIcon.kt	75.00% <66.66%> (+15.00%)	0.00 <0.00> (ø)
...pp/src/main/java/org/mozilla/fenix/ext/TextView.kt	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...iewholders/PrivateBrowsingDescriptionViewHolder.kt	100.00% <100.00%> (+100.00%)	1.00 <0.00> (+1.00)
...ers/onboarding/OnboardingManualSignInViewHolder.kt	100.00% <100.00%> (+7.69%)	2.00 <0.00> (ø)
...holders/onboarding/OnboardingWhatsNewViewHolder.kt	100.00% <100.00%> (+100.00%)	1.00 <0.00> (+1.00)
...nix/trackingprotectionexceptions/ExceptionsView.kt	100.00% <100.00%> (ø)	4.00 <0.00> (ø)
...va/org/mozilla/fenix/components/metrics/Metrics.kt	21.62% <0.00%> (+0.27%)	0.00% <0.00%> (ø%)
... and 3 more

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