Store previous leading icon state to preserve Easter egg logos on focus change

When the omnibar gains focus, the leading icon changes to Search. Previously,
if an Easter egg logo was displayed, it would be lost when focus was regained
because the logic only checked the current leading icon state (which was Search).

Changes:
- Added previousLeadingIconState field to ViewState to store icon state before focus
- Store current leading icon state when gaining focus
- Use previous state (instead of current) when losing focus to restore Easter egg logos
- Clear previous state after restoration to avoid memory retention

This ensures Easter egg logos persist correctly through focus changes.

Author: mikescamell

app/src/main/java/com/duckduckgo/app/browser/omnibar/OmnibarLayoutViewModel.kt

@@ -152,6 +152,7 @@ class OmnibarLayoutViewModel @Inject constructor(
     data class ViewState(
         val viewMode: ViewMode = Browser(null),
         val leadingIconState: LeadingIconState = Search,
+        val previousLeadingIconState: LeadingIconState? = null,
         val privacyShield: PrivacyShieldState = PrivacyShieldState.UNKNOWN,
         val hasFocus: Boolean = false,
         val query: String = "",
@@ -286,6 +287,7 @@ class OmnibarLayoutViewModel @Inject constructor(
                     hasFocus = true,
                     expanded = true,
                     leadingIconState = Search,
+                    previousLeadingIconState = it.leadingIconState,
                     highlightPrivacyShield = HighlightableButton.Gone,
                     showClearButton = showClearButton,
                     showTabsMenu = showControls,
@@ -322,15 +324,16 @@ class OmnibarLayoutViewModel @Inject constructor(
                     it.omnibarText
                 }
 
-                val currentLogoUrl = when (val leadingIconState = it.leadingIconState) {
-                    is EasterEggLogo -> leadingIconState.logoUrl
+                val currentLogoUrl = when (val previousState = it.previousLeadingIconState) {
+                    is EasterEggLogo -> previousState.logoUrl
                     else -> null
                 }
 
                 it.copy(
                     hasFocus = false,
                     expanded = false,
                     leadingIconState = getLeadingIconState(false, it.url, currentLogoUrl),
+                    previousLeadingIconState = null,
                     highlightFireButton = HighlightableButton.Visible(highlighted = false),
                     showClearButton = false,
                     showTabsMenu = true,