Smallest changes needed to fix the issue from Bug 1815421

Bug 1815421 - AddLoginFragment.patch

Index: fenix/app/src/main/java/org/mozilla/fenix/settings/logins/fragment/AddLoginFragment.kt
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/fenix/app/src/main/java/org/mozilla/fenix/settings/logins/fragment/AddLoginFragment.kt b/fenix/app/src/main/java/org/mozilla/fenix/settings/logins/fragment/AddLoginFragment.kt
--- a/fenix/app/src/main/java/org/mozilla/fenix/settings/logins/fragment/AddLoginFragment.kt	(revision d4fdfa701d64b5861e7da820c47be344ffade347)
+++ b/fenix/app/src/main/java/org/mozilla/fenix/settings/logins/fragment/AddLoginFragment.kt	(date 1677516765310)
@@ -50,7 +50,7 @@
     private var duplicateLogin: SavedLogin? = null
 
     private var validPassword = false
-    private var validUsername = true
+    private var validUsername = false
     private var validHostname = false
     private var usernameChanged = false
 
@@ -198,9 +198,7 @@
         binding.usernameText.addTextChangedListener(
             object : TextWatcher {
                 override fun afterTextChanged(editable: Editable?) {
-                    // update usernameChanged to true when the text is not empty,
-                    // otherwise it is not changed, as this screen starts with an empty username.
-                    usernameChanged = editable.toString().isNotEmpty()
+                    usernameChanged = true
                     updateUsernameField()
                     setSaveButtonState()
                     findDuplicate()
@@ -297,7 +295,7 @@
                     ),
                 )
             }
-            else -> {
+            usernameChanged -> {
                 // Valid username
                 validUsername = true
                 layout.error = null