Move the app to /system_ext

As mentioned in go/partitions-in-r, we plan to forbid hidden API in
product partition.
So this app should be moved to /system_ext because it uses hidden API.
It is not permanent, so they can be moved back to /product due to
needs if there isn't hidden usage anymore.

Bug: 137908189
Test: m
Test: check whether device boots successfully
Change-Id: Icd2f7fc7021e84fad9980d58688dc640ca4245f8

Android.bp

@@ -50,7 +50,7 @@ android_app {
     name: "Settings",
     platform_apis: true,
     certificate: "platform",
-    product_specific: true,
+    system_ext_specific: true,
     privileged: true,
     required: ["privapp_whitelist_com.android.settings"],
     static_libs: ["Settings-core"],