Enable platform views for Flutter Gallery on iOS. (#29632)

This results in running Flutter Gallery with one less thread (platform
and gpu threads are the the same).

This is very likely to cause a regression in the iOS Gallery benchmarks.
I'm mainly interested in landing this to see how much the benchmarks
regress, and it's likely that we will revert it shortly after landing.

examples/flutter_gallery/ios/Runner/Info.plist

@@ -41,6 +41,8 @@
 	</array>
 	<key>ITSAppUsesNonExemptEncryption</key>
 	<false/>
+	<key>io.flutter.embedded_views_preview</key>
+	<true/>
 	<key>UIViewControllerBasedStatusBarAppearance</key>
 	<false/>
 </dict>