Remove ElementKey.isBackground (1/2)

This CL is a small refactor that completely removes the
ElementKey.isBackground modifier, and instead introduces a
LowestZIndexScenePicker that can be used to draw background in the scene
with lowest zIndex.

Bug: 317026105
Test: ElementScenePickerTest
Flag: N/A
Change-Id: I61cbc6010f50d839cf7046a97e17afcecc48556c
4 files changed