[flexiglass] Defer reading QS squishiness

Defers the consumption of "squishiness" to later by passing lambdas
around instead of the actual value. This gets rid of the crash of the
associated bug.

Fix: 341061899
Test: manually verified that the reproduction steps from comment #6 on
the bug no longer cause a crash
Flag: com.android.systemui.scene_container

Change-Id: I4751671c76fa7b3b37cdcb3a8028fec43aea7172