[fenix] Fix for https://github.com/mozilla-mobile/fenix/issues/8430 - Migration progress bar too close to text

Reduced the margin start value of the progress bar in order to leave more space between it and the
text.

Moved hardcoded margin values into dimens resources.

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

@@ -104,9 +104,9 @@
         style="@style/Widget.AppCompat.ProgressBar"
         android:layout_width="@dimen/migration_progress_size"
         android:layout_height="@dimen/migration_progress_size"
-        android:layout_marginStart="8dp"
-        android:layout_marginTop="8dp"
-        android:layout_marginBottom="8dp"
+        android:layout_marginStart="@dimen/migration_progress_margin_start"
+        android:layout_marginTop="@dimen/migration_progress_margin_vertical"
+        android:layout_marginBottom="@dimen/migration_progress_margin_vertical"
         app:layout_constraintBottom_toBottomOf="@+id/migration_button"
         app:layout_constraintStart_toStartOf="@+id/migration_button"
         app:layout_constraintTop_toTopOf="@+id/migration_button" />

app/src/main/res/values/dimens.xml

@@ -112,5 +112,7 @@
     <dimen name="migration_description_text_size">16sp</dimen>
     <dimen name="migration_welcome_title_text_size">20sp</dimen>
     <dimen name="migration_progress_size">24dp</dimen>
+    <dimen name="migration_progress_margin_vertical">8dp</dimen>
+    <dimen name="migration_progress_margin_start">4dp</dimen>
 
 </resources>