Fix invisible Find Previous & Find Next buttons

Move the corresponding icon assets to the DesginSystem module so that
they can be found at runtime.

Fixes #3699

focus-ios/Blockzilla/Utilities/FindInPageBar.swift

@@ -77,14 +77,14 @@ class FindInPageBar: UIView {
         matchCountView.accessibilityIdentifier = "FindInPage.matchCount"
         addSubview(matchCountView)
 
-        previousButton.setImage(UIImage(named: "find_previous"), for: [])
+        previousButton.setImage(.findPrevious, for: [])
         previousButton.setTitleColor(.white, for: [])
         previousButton.accessibilityLabel = UIConstants.strings.findInPagePreviousLabel
         previousButton.addTarget(self, action: #selector(didFindPrevious), for: .touchUpInside)
         previousButton.accessibilityIdentifier = "FindInPage.find_previous"
         addSubview(previousButton)
 
-        nextButton.setImage(UIImage(named: "find_next"), for: [])
+        nextButton.setImage(.findNext, for: [])
         nextButton.setTitleColor(.white, for: [])
         nextButton.accessibilityLabel = UIConstants.strings.findInPageNextLabel
         nextButton.addTarget(self, action: #selector(didFindNext), for: .touchUpInside)

focus-ios/BlockzillaPackage/Sources/DesignSystem/UIImage+AppImages.swift

@@ -46,4 +46,8 @@ public extension UIImage {
     static let delete = UIImage(named: "icon_delete")!
     static let hamburgerMenu = UIImage(named: "icon_hamburger_menu")!
     static let stopMenu = UIImage(named: "icon_stop_menu")!
+
+    // MARK: Find In Page
+    static let findPrevious = UIImage(named: "find_previous")!
+    static let findNext = UIImage(named: "find_next")!
 }