Fix a few things regarding choices and actions

[v-kydela]

• Modify ChoiceFactory.HeroCard to incorporate each choice's Action property the way ChoiceFactory.SuggestedAction does
• Centralize that functionality in a shared method
• Update the Choice.Action doc comment to reflect that
• Add tests in ChoiceFactoryTests to account for actions in hero cards and suggested actions
• Add basic test for ChoiceFactory.HeroCard that was missing

This is meant to piggyback on garypretty's recent PR since I noticed Choice.Action wasn't incorporated into ChoiceFactory.HeroCard: #1339

[v-kydela]

I want to note that at some point I think ChoiceFactoryTests should be updated with more tests to account for every channel. This could be made easier if some private methods were included to account for the code that keeps getting repeated across the tests. I also notice that the JS repo has some choice factory tests that this repo does not.

[coveralls]

Pull Request Test Coverage Report for Build 48584

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 72.176%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs/Choices/ChoiceFactory.cs	7	79.7%

Totals
Change from base Build 48579:	0.01%
Covered Lines:	3738
Relevant Lines:	5179

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💛 - Coveralls