Prevent unnecessary recompositions on communal container

This change addresses a performance regression that caused unnecessary
recompositions on the communal container when expanding and collapsing
the shade over the lockscreen. This was solved by moving the relevant
state changes to a lower, more specific composable level, making sure
that the recompositions only occur when the communal scene is present.

Test: verified communal container does not recompose when expanding and
      collapsing the shade over lockscreen
Test: verified touches on the blank spaces in the shade does not fall
      through to the communal scene
Test: verified pulling down the shade over communal scene causes the
      scene to blur correctly
Bug: 432615057
Flag: EXEMPT improve performance
Change-Id: I55eb81c329432119a0e62089e293a03d343088ef