Update `AccountActivity` to properly handle external Intent

[cketti]

Currently the internal and external Intent extra mechanism both use the key account. This leads to AccountIdIntentSerializer always returning a LegacyAccount in the case where a third-party app uses the documented way to open a specific account (see https://manual.davx5.com/integration.html#open-davx5-for-a-specific-account). However, if the given Android account is linked to a database account we need to use a DbAccountId instance internally.

To fix this we should…

• use a different extra key for internal navigation, e.g. accountId (instead of account),
• use AndroidAccountManager to retrieve the proper AccountId associated with the given Android account when handling third-party intents (account extra).

Note: Since this lookup in AndroidAccountManager involves a database access and we don't want to do this in the main thread, we either need to defer showing AccountScreen until the AccountId is available or change AccountScreen to support a loading state.