Fix media viewer for medias not related to events displaying invalid state (#7280)

* When no URL is provided to `ContentValidationState.collectMediaState`, assume it's valid

That way we can do:

`if (validationState.isValid() && state.collectMediaState(possiblyNullUrl).isValid()) ...`

* Remove the `eventId` param from `ContentScannerService.scan`

It's not really used, since we then provide a state that can be mutated, and fetching that is what the previous parameter was used for

* Make sure to always validate the medias in the viewer, both the actual media and the thumbnail

* Update screenshots

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Co-authored-by: ElementBot <android@element.io>

libraries/matrixui/src/main/kotlin/io/element/android/libraries/matrix/ui/media/contentvalidation/ContentValidationState.kt

@@ -189,7 +189,7 @@ fun ContentValidationState.collectOverallState(): State<ContentValidationValue>
 @Composable
 fun ContentValidationState.collectMediaState(url: String?): State<ContentValidationValue> {
     if (url == null) {
-        return remember { mutableStateOf(ContentValidationValue.Unknown) }
+        return remember { mutableStateOf(ContentValidationValue.Valid) }
     }
 
     return produceState(getCurrentMediaState(url), key1 = url) {