Move sysui QS user detail config to feature flag

This replaces config_keyguard_user_switch_opens_qs_details in config.xml
with flag_lockscreen_qs_user_detail_shortcut in flags.xml

Replacing the config with a feature flag allows the UX to be toggled on
and off using adb.

Bug: 194237078
Test: adb shell setprop \
	persist.systemui.flag_lockscreen_qs_user_detail_shortcut 1
Change-Id: I99d224137c23a41de0bda1c40ff5622e267347b2