Added in Online Accounts: But still prompt for password

## What Happened

When Mail is packaged using Flatpak, it is not capable of retrieving the before configured secrets from Online Accounts. This leads to a password prompt when Mail is freshly started after adding a new account in System Settings -or as @danrabbit puts it in [his comment](https://github.com/elementary/switchboard-plug-onlineaccounts/pull/184#pullrequestreview-676513515):

> I'm still having trouble with the saved password in the flatpak, but this does seem to work for native mail. I wonder if there's a dbus interface we're missing that we need to add to the Mail sandbox

## Expected Behavior

Once the account is added in Online Accounts, it should be fully usable from Mail - without forcing the user to retype his password again.

## Steps to Reproduce

In the latest elementary OS Beta:

1. Add a new Mail account through `System Settings > Online Accounts`
2. Open Mail
3. A password prompt appears

## Potential solution

as @Marukesu [points out](https://github.com/elementary/switchboard-plug-onlineaccounts/pull/184#issuecomment-855288992), we are probably dealing with a secrets management issue in Flatpak-Land:

> eds use libsecret to read/save passwords, because we are on a sandbox, libsecret try to get them from a file on the sandbox instead of the DBus Interface.

Regarding [this](https://opensource.com/article/19/11/secrets-management-flatpak-applications) article, we might want fall back to the legacy behaviour by allowing access to the Systems Secrets Service API via DBus (`org.freedesktop.secrets`). But I'm wondering, if there is a more elegant solution to this matter, since this solution puts a hole in the Flatpak sandbox.