refactor: migrate main activity layout to ConstraintLayout

This commit replaces the root `LinearLayout` with `ConstraintLayout` in `activity_main.xml`. It adds the necessary constraint attributes to the child views (AdView, content frame, and the flash controls RelativeLayout) to maintain the layout structure.

Author: eby-dev

app/src/main/res/layout/activity_main.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/layout_activity_main"
@@ -14,19 +14,29 @@
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         app:adSize="BANNER"
-        app:adUnitId="@string/AdMob_Banner_Ads_ID" />
+        app:adUnitId="@string/AdMob_Banner_Ads_ID"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        app:layout_constraintTop_toTopOf="parent" />
 
     <FrameLayout
         android:id="@+id/content_frame"
         android:layout_width="match_parent"
         android:layout_height="0dp"
-        android:layout_weight="1" />
+        android:layout_weight="1"
+        app:layout_constraintBottom_toBottomOf="parent"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent"
+        app:layout_constraintTop_toBottomOf="@id/adView" />
 
     <RelativeLayout
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_gravity="center"
-        android:paddingBottom="5dp">
+        android:paddingBottom="5dp"
+        app:layout_constraintBottom_toBottomOf="parent"
+        app:layout_constraintEnd_toEndOf="parent"
+        app:layout_constraintStart_toStartOf="parent">
 
         <ImageView
             android:id="@+id/flashOff"
@@ -45,4 +55,4 @@
             android:src="@drawable/ic_flash_on" />
     </RelativeLayout>
 
-</LinearLayout>
+</androidx.constraintlayout.widget.ConstraintLayout>