Remove dynamic summaries for "Screen lock"

By replacing default summary string with placeholder in xml, the search
index db will add the placeholder as summary instead. This prevents
SettingsIntelligence displaying the search result with a fixed summary,
which may inconsistent with current settings status.

Fixes: 73254496
Test: RunSettingsRoboTests
Change-Id: Ifae8a787ce63fa7a6c7ad5749dab7db72be0263f