280 fix problematic toasts (bitfireAT/davx5#283)

* When renaming show a toast when account with new name exists already

* Replace toasts with snackbars

* Apply code styling hints

* Move lambdas out of parentheses

* Inject application instead of context

* Replace Toast with Snackbar

* Shorten messages shown in snackbars

* Show Toast in UI instead of model; use AndroidViewModel

* Move account name check out of lock; don't close activity if not successful

* Duplicate account name check: only check for our account type

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Co-authored-by: Ricki Hirner <hirner@bitfire.at>