Move icu related hidden API flag file contents to icu

Bug: 179354495
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: I965651e0d88b759f023620174be3c2fbd19cb31e
diff --git a/boot/Android.bp b/boot/Android.bp
index 0fc455b..ef2abc8 100644
--- a/boot/Android.bp
+++ b/boot/Android.bp
@@ -51,6 +51,10 @@
             apex: "com.android.art",
             module: "art-bootclasspath-fragment",
         },
+        {
+            apex: "com.android.i18n",
+            module: "i18n-bootclasspath-fragment",
+        },
     ],
 
     // Additional information needed by hidden api processing.