Expose SYSTEM_UPGRADE_SETTINGS as a public API

This change exposes android.settings.SYSTEM_UPDATE_SETTINGS as a public
API, enabling applications to directly navigate users to system upgrade
page in a secure and standardized manner.

Many OEMs have implemented the API in advance for years. There is no
additional implementation effort in AOSP.

CTS test cases will be introduced to enforce all OEMs to support the
contract of the API.

Bug: 417458670
Test: atest android.provider.cts.settings.SettingsTest
Flag: android.provider.expose_system_update_settings
Change-Id: I193895a5a8571c02a836c8c219bc26909f079ec3