Update "Time spent in app" to "Screen time"

    <string name="app_usage_preference">App usage preferences</string>

    <!-- Link title to show stats about how much time user spent in an app [CHAR LIMIT=45] -->

    <string name="time_spent_in_app_pref_title">Time spent in app</string>

    <string name="time_spent_in_app_pref_title">Screen time</string>

    <!-- Description of the usage access setting [CHAR LIMIT=NONE] -->

    <string name="usage_access_description">Usage access allows an app to track what other apps you\u2019re using and how often, as well as your carrier, language settings, and other details.</string>