Fixed elapsed_time_millis in SettingsUIChanged event

This change stores timestamp when received com.google.android.setupwizard.SETUP_WIZARD_FINISHED. This timestamp will be used to calculate elapsed_time_millis for SettingsUIChanged event after SUW complete. This enables to analyze how the user uses Settings in a specific time span after setup.

Bug: 344466251
Test: metrics related change only
Flag: EXEMPT metrics change only

Change-Id: I85b15f1eb5e5a4502a27d8588bb01e59b7ad83b5