Remove redundant content in javadoc of new introduced API

This change removes the implementation suggestion for the javadoc of API ACTION_SYSTEM_UPDATE_SETTINGS. Javadoc of an API should be client-oriented, not implementation-oriented.

The implementation suggestion is enforced by introducing new CTS test cases to avoid security issue.

Bug: 417458670
Fix: 423988266
Test: atest android.provider.cts.settings.SettingsTest
Flag: android.provider.expose_system_update_settings
Change-Id: Icf63d9f36b2ffe203429e1c8a4aa02f58b49ca21