[Bug] Microsoft.Identity.Web should not call GetAccountsAsync()

[jmprieur]

Which Version of Microsoft Identity Web are you using ?
Note that to get help, you need to run the latest version.

Where is the issue?
B2C scenario

Other? - please describe;

Repro
TokenAcquisition calls GetAccountsAsync in the case of B2C to remove the accounts. However, this will call the token cache serialiation with no suggested cache key.

https://github.com/AzureAD/microsoft-identity-web/search?q=GetAccountsAsync&unscoped_q=GetAccountsAsync

Expected behavior
When signing out, the token for a user are removed from the cache

Actual behavior
The cache is not cleared as no account is found

Possible Solution

• We should probably think of adding all the tokens for all the policies of a given account in the same token cache, which we could clear at once.
• then we should not call GetAccountsAsync(), but GetAccountAsync(account-without-policy)

Additional context/ Logs / Screenshots
Add any other context about the problem here, such as logs and screenshots.

[jennyf19]

Part of this pr

[pmaytak]

Included in Microsoft Identity Web 0.2.1-preview release.