Eanble-AzContextAutoSave should save AzureRmContext.json if default context not empty

## Description of the new feature
For cmdlets below, we expect to same output of `Get-AzContext #2` as `Get-AzContext #1`, but actually it does not. The root cause is `Enable-AzContextAutosave` only save in-memory token to disk, but doesn't save in-memory context to AzureRmContext.json

```powershell
Clear-AzContext
Disable-AzContextAutosave
Connect-AzAccount
Get-AzContext #1
Enable-AzContextAutosave
Get-AzContext #2
```

## Proposed implementation details (optional)

Reproduce the sequence using Clear-AzContext, Disable-AzContextAutosave, Connect-AzAccount, Get-AzContext, and Enable-AzContextAutosave. Trace the Enable-AzContextAutosave cmdlet and context persistence behavior; done means a non-empty default context is saved to AzureRmContext.json and Get-AzContext #2 matches Get-AzContext #1.

Written by the indexing model from the issue text.