[fenix] For https://github.com/mozilla-mobile/fenix/issues/4262: Updates color and text of buttons (https://github.com/mozilla-mobile/fenix/pull/4250)

app/src/main/res/drawable/button_background.xml

@@ -5,5 +5,5 @@
 
 <shape xmlns:android="http://schemas.android.com/apk/res/android">
     <corners android:radius="4dp"/>
-    <solid android:color="?inset" />
+    <solid android:color="?accent" />
 </shape>

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

@@ -16,9 +16,9 @@
     android:letterSpacing="0"
     android:text="@string/save_to_collection"
     android:textAllCaps="false"
-    android:textColor="?primaryText"
+    android:textColor="?contrastText"
     app:backgroundTint="?accent"
     app:icon="@drawable/ic_tab_collection"
     app:iconGravity="textStart"
     app:iconPadding="8dp"
-    app:iconTint="?attr/primaryText" />
+    app:iconTint="?attr/contrastText" />