Closes #20901: Do not record viewTime observations when we do not have a set lastAccess

The bug here was that we'd try to record `now - 0` as a viewTime delta.
This isn't just an obviously wrong value to record, but it will also
overflow our storage - we'll end up with a value on disk that doesn't
fit into an i32, but HistoryMetadata.total_view_time is i32 in our Rust
struct. Once that happens, reads that touch this bad row will result in
an overflow and a crash.

app/src/main/java/org/mozilla/fenix/historymetadata/HistoryMetadataService.kt

@@ -81,11 +81,17 @@ class DefaultHistoryMetadataService(
     }
 
     override fun updateMetadata(key: HistoryMetadataKey, tab: TabSessionState) {
-        logger.debug("Updating metadata for tab $tab")
+        val lastAccess = tab.lastAccess
+        if (lastAccess == 0L) {
+            logger.debug("Not updating metadata for tab $tab - lastAccess=0")
+            return
+        } else {
+            logger.debug("Updating metadata for tab $tab")
+        }
 
         scope.launch {
             val viewTimeObservation = HistoryMetadataObservation.ViewTimeObservation(
-                viewTime = (System.currentTimeMillis() - tab.lastAccess).toInt()
+                viewTime = (System.currentTimeMillis() - lastAccess).toInt()
             )
             storage.noteHistoryMetadataObservation(key, viewTimeObservation)
         }