[flexiglass] Adds long-press settings menu support to lockscreen scene.

Bug: 280879610
Test: manually verified that long-pressing on empty spot in the
lockscreen shows the lockscreen settings menu and clicking it opens the
wallpaper picker. Manually verified the touching outside the settings
menu dismisses it.

Change-Id: I49c77e9073a280716b05a3ef22c4044a4af258f1