Reverts "[gallery] Reland roll gallery to ecfb9e5352bd12032301b12b30d5853d83d89bda" (#137264)

Reverts flutter/flutter#137199
Initiated by: zanderso
This change reverts the following previous change:
Original Description:
Reverts flutter/flutter#133095

Dependencies have been fixed, hopefully : https://github.com/flutter/flutter/issues/133096#event-10761117209

dev/devicelab/lib/versions/gallery.dart

@@ -3,4 +3,4 @@
 // found in the LICENSE file.
 
 /// The pinned version of flutter gallery, used for devicelab tests.
-const String galleryVersion = 'ecfb9e5352bd12032301b12b30d5853d83d89bda';
+const String galleryVersion = '6a8d738c94d0710e229d726729c09fdb5ccaf7ed';