Flexiglass: add SceneStack.toString

I'd like to print the back stack in a debug message in a future change,
but it didn't have a toString, so I'm implementing one.

Bug: 364076359
Test: builds
Flag: com.android.systemui.scene_container
Change-Id: I6d048a4fe8f79c3e11977a25dfa0b29074cf79cd
3 files changed