Add a flag for switching to use snippet client v2 - Simpler Version

[mhaoli]

This PR is a part of #793.

This PR adds a flag for switching to use snippet client v2, including:

• Adds a command line argument use_mobly_snippet_client_v2.
• If the arg is specified, the BaseTest class will directly set the flag to SnippetManagementService, which is responsible for creating, stopping and resuming snippet client.

For this switch, this PR only sets SnippetManagementService._use_client_v2 to True instead of really using snippet client v2, because client v2 is not ready now.

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This change is 

[xpconanfan]

Discussed offline. Try a simpler approach