Have StringPreference also persist a default if no value currently exist

[Mugurell]

Saw the need on Fenix to use a locally persisted string but having to first provide a value and ensuring this is fist persisted to then always be returned on subsequent requests.
This could be done manually in-place but a more general approach supported by default in StringPreference can also be extracted.

Confirmed with @Amejia481 an updated api that would add a new parameter:

persistDefaultIfNotExists: Boolean = false

which would mean no breaking changes
but an easy way to add the new behavior for the full api now being:

 var persistedStringValue by stringPreference(
      key = "permissions_enabled",
      default = "blocked",
      persistDefaultIfNotExists = true,
 )

┆Issue is synchronized with this Jira Task

[Mugurell]

Change landed and is used on Fenix. Closing.