[flexiglass] Wire durationScale properly for transition builders.

We weren't passing in the durationScale and/or were overriding it inside
another function inside the main function.

Test: manually verified that Flexiglass scene transitions when locked
and unlocked still look correct.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Bug: 330672236

Change-Id: Ic9b01ed8fa2bbe4edd0ad75e9e9a854709bd10dd