[PORT]Add configuration for default languagePolicy

[Danieladu]

close: #2028
Composer requires a customized language policy for language fallback, than the default "en-us" => "en" => "".

In composer scenario, we want be able to fallback to specific language instead of empty, for example fallback to en-us when activity.locale = fr-fr, because the bot is only configured with en-us assets.

This change include

1. Replace the lazy trick in LanguagePolicy by a class singleton, to make sure json deserialization work.
2. Register the root dialog's policy as default one in TurnContext, if children dialog don't configured with a policy, it will pick that one.

[coveralls]

Pull Request Test Coverage Report for Build 138552

• 24 of 26 (92.31%) changed or added relevant lines in 5 files are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 80.832%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botbuilder-dialogs-adaptive/src/generators/multiLanguageGeneratorBase.ts	12	13	92.31%
libraries/botbuilder-dialogs-adaptive/src/languageGenerationMiddleware.ts	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%
libraries/botbuilder-dialogs-adaptive/src/languageResourceLoader.ts	8	69.34%

Totals
Change from base Build 138551:	-0.1%
Covered Lines:	12821
Relevant Lines:	15144

---

💛 - Coveralls