sdk-extensions-info.xml: add the U SDK extensions (U-ext)

Introduce the <sdk> tag for the U SDK extensions.

Also update README.md to mention that a new <sdk> tag needs to be added
when a new SDK extension is created.

Bug: b/278833947
Test: m sdk dist
Change-Id: I867ab78d706c550117bccdd4fb27bbf283380f24

README.md

@@ -156,3 +156,5 @@ The steps to define a new extension are:
     *   Extend `CtsSdkExtentensionsTestCase` to verify the above two behaviors.
 
     *   e.g. [for U extensions](http://ag/21507939)
+
+-   Add a new sdk tag in sdk-extensions-info.xml

sdk-extensions-info.xml

@@ -36,6 +36,11 @@
     shortname="T-ext"
     name="T Extensions"
     reference="android/os/Build$VERSION_CODES$TIRAMISU" />
+  <sdk
+    id="34"
+    shortname="U-ext"
+    name="U Extensions"
+    reference="android/os/Build$VERSION_CODES$UPSIDE_DOWN_CAKE" />
   <sdk
     id="1000000"
     shortname="AD_SERVICES-ext"
@@ -46,31 +51,31 @@
   <symbol
     jar="framework-sdkextensions"
     pattern="*"
-    sdks="R-ext,S-ext,T-ext" />
+    sdks="R-ext,S-ext,T-ext,U-ext" />
 
   <!-- MEDIA_PROVIDER -->
   <symbol
     jar="framework-mediaprovider"
     pattern="android.provider.MediaStore.EXTRA_PICK_IMAGES_MAX"
-    sdks="R-ext,S-ext,T-ext" />
+    sdks="R-ext,S-ext,T-ext,U-ext" />
   <symbol
     jar="framework-mediaprovider"
     pattern="android.provider.MediaStore.ACTION_PICK_IMAGES"
-    sdks="R-ext,S-ext,T-ext" />
+    sdks="R-ext,S-ext,T-ext,U-ext" />
   <symbol
     jar="framework-mediaprovider"
     pattern="android.provider.MediaStore.getPickImagesMaxLimit"
-    sdks="R-ext,S-ext,T-ext" />
+    sdks="R-ext,S-ext,T-ext,U-ext" />
   <symbol
     jar="framework-mediaprovider"
     pattern="android.provider.MediaStore.PickerMediaColumns"
-    sdks="R-ext,S-ext,T-ext" />
+    sdks="R-ext,S-ext,T-ext,U-ext" />
 
   <!-- CONNECTIVITY -->
   <symbol
     jar="framework-connectivity"
     pattern="android.net.http"
-    sdks="S-ext,T-ext" />
+    sdks="S-ext,T-ext,U-ext" />
 
   <!-- AD_SERVICES -->
   <!--