[flexiglass] Long-press settings menu in Compose-based LockscreenScene.

Makes the settings menu/popup show when long-pressed and dismiss when
touched outside.

Bug: 316211368
Test: manually verified long-pressing brings up the popup in the right
place
Test: manually verified that the popup automatically hides after a few
seconds if I do nothing
Test: manually verified that touching in the popup correctly opens WPP
Test: manually verified that touching outside dismisses the popup
Test: manually verified all of the above also when UseLockscreenContent
is false
Test: manually verified all of the above with Flexiglass off
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I0b3acc04d04bf21088b2806fc8094abea3da668a