[flexiglass] Fixes clock and smartspace jitter in AOD

The problem in the old logic was that viewModel.updateBurnInParams was
called on every recomposition of the burnInAware @Composable function.
In turn, that caused the ViewModel to emit a brand new value onto its
movement flow which then would cause another recomposition of
burnInAware, repeating the cycle.

That explains why the jitter happens repeatedly but it doesn't explain
why the jitter (a rapid repositioning of the elements on the Y axis in two
location).

I'm not sure why the jitter happens but I must assume that it's some
translation animation that's handled in the Views as we enter and exit
AOD. The cycle described above simply retriggers it on every frame.

The cycle is broken by only calling viewModel.updateBurnInParams once
each time the burnInAware composable enters the composition using a
LaunchedEffect(Unit). Upon closer inspection, one can see that the
updateBurnInParams call doesn't really need to instantiate a new params
object every time that translationYState changed as it just provides a
copied params object that has a translationY getter function and that
function is bound to returning the value of translationYState - so it
only needs to happen once.

Fix: 371211911
Test: manually verified that entering and exiting AOD repeatedly doesn't
cause the clock and smartspace to jitter. They just animate vertically
very nicely as they should.
Flag: com.android.systemui.scene_container

Change-Id: I9e3de5f8d7f0dbe4e67ce5e70044fea77f58fff3