Fix Lint warnings when using `android:textAlignment="center"`

Use `android:gravity="center_horizontal"`

Test: ./run_all_tests.sh

NavigationAdvancedSample/app/src/main/res/layout/fragment_about.xml

@@ -27,6 +27,7 @@
 
     <TextView
         android:id="@+id/about_tv"
+        android:gravity="center_horizontal"
         android:layout_width="368dp"
         android:layout_height="87dp"
         android:layout_marginStart="8dp"

NavigationAdvancedSample/app/src/main/res/layout/fragment_registered.xml

@@ -26,6 +26,7 @@
     tools:layout_editor_absoluteY="81dp">
 
     <TextView
+        android:gravity="center_horizontal"
         android:layout_width="368dp"
         android:layout_height="81dp"
         android:layout_marginStart="8dp"

NavigationAdvancedSample/app/src/main/res/layout/fragment_title.xml

@@ -25,6 +25,7 @@
     tools:layout_editor_absoluteY="81dp">
 
     <TextView
+        android:gravity="center_horizontal"
         android:id="@+id/game_title"
         android:layout_width="368dp"
         android:layout_height="81dp"

NavigationBasicSample/app/src/main/res/layout/fragment_game_over.xml

@@ -28,6 +28,7 @@
 
     <TextView
         android:id="@+id/game_title3"
+        android:gravity="center_horizontal"
         android:layout_width="368dp"
         android:layout_height="97dp"
         android:layout_marginStart="8dp"

NavigationBasicSample/app/src/main/res/layout/fragment_match.xml

@@ -74,6 +74,7 @@
 
     <TextView
         android:id="@+id/textView"
+        android:gravity="center_horizontal"
         android:layout_width="59dp"
         android:layout_height="46dp"
         android:layout_marginStart="8dp"

NavigationBasicSample/app/src/main/res/layout/fragment_results_winner.xml

@@ -28,6 +28,7 @@
 
     <TextView
         android:id="@+id/game_title2"
+        android:gravity="center_horizontal"
         android:layout_width="368dp"
         android:layout_height="53dp"
         android:layout_marginStart="8dp"