Previous code could crash using:

```kotlin
private val activeAccount = accountManager.activeAccount!!

...

fun setAccountInfo(accountId: String) {
    this.accountId = accountId
    // NPE on next line
    this.isSelf = activeAccount.accountId == accountId
    reload(false)
}
```

Fix this by starting to refactor `AccountViewModel` to expose all of
this information as flows with correct dependencies between them.

Not all observables are converted to flows, only the minimum needed to
fix this problem. Future work will convert the other observables and
update the `AccountViewModel` design.

Concrete changes:

- `AccountActivity.loadedAccount` is gone, `AccountViewModel.accountData`
is created and collected. This emits a
`Result<Loadable.Loaded<Account>, GetAccountError>` to handle failure
cases.
- Functions that need the account (like `updateAccountAvatar()`) now
take the account as a parameter, instead of reading it from a nullable
`loadedAccount`.
- Similar changes for `AccountViewModel.isSelf`. This is now a stateflow
allowing code to react to it changing while also using the current value
in code that can't react.
- Create `AccountRepository` and related classes to start holding code
that interacts with accounts. Make this injectable with Hilt.