Fill area under message entry with same bg color as message entry

With recent changes for SC 2.0 design, area under message entry must be of the same color as message entry background color.

MOB-3889

GliaWidgets/Sources/Theme/Theme+Chat.swift

@@ -350,7 +350,7 @@ extension Theme {
             placeholderFont: font.bodyText,
             placeholderColor: color.baseShade,
             separatorColor: color.baseShade,
-            backgroundColor: color.baseLight,
+            backgroundColor: color.baseNeutral,
             mediaButton: mediaButton,
             sendButton: sendButton,
             uploadList: uploadListStyle,

GliaWidgets/Sources/View/Chat/ChatView.DefineLayout.swift

@@ -96,5 +96,13 @@ extension ChatView {
 
         bringSubviewToFront(entryWidgetOverlayView)
         bringSubviewToFront(entryWidgetContainerView)
+
+        constraints += [
+            messageEntryBottomArea.leadingAnchor.constraint(equalTo: safeAreaLayoutGuide.leadingAnchor),
+            messageEntryBottomArea.trailingAnchor.constraint(equalTo: safeAreaLayoutGuide.trailingAnchor),
+            messageEntryBottomArea.topAnchor.constraint(equalTo: messageEntryView.bottomAnchor),
+            messageEntryBottomArea.bottomAnchor.constraint(lessThanOrEqualTo: bottomAnchor)
+        ]
+        insertSubview(messageEntryBottomArea, at: 0)
     }
 }

GliaWidgets/Sources/View/Chat/ChatView.swift

@@ -31,6 +31,15 @@ class ChatView: EngagementView {
     let secureMessagingBottomBannerView = SecureMessagingBottomBannerView().makeView()
     let sendingMessageUnavailabilityBannerView = SendingMessageUnavailableBannerView().makeView()
 
+    // Instead of modifying message entry view's layout to resize, covering bottom safe area,
+    // thus affecting existing layout calculations, we add additional view just for that,
+    // keeping background color for it reactively in sync with message entry view's one.
+    lazy var messageEntryBottomArea = UIView().makeView { [weak messageEntryView] area in
+        messageEntryView?.publisher(for: \.backgroundColor).sink { [weak area] newColor in
+            area?.backgroundColor = newColor
+        }.store(in: &cancelBag)
+    }
+
     let style: ChatStyle
     let environment: Environment