[flexiglass] Use LaunchedEffectWithLifecycle to detect ShadeMode changes

With `alwaysCompose` enabled for Quick Settings, the activatable scope
is no longer limited to the Quick Settings shade being shown. Therefore
we switch to LaunchedEffectWithLifecycle instead, to ensure ShadeMode
detection only runs while the shade is expanded.

Fix: 438770974
Test: Updated unit tests.
Test: Manually tested by disabling and re-enabling Dual Shade in the
 Settings app, and observing that Quick Settings no longer unexpectedly
 opens.
Flag: com.android.systemui.scene_container
Change-Id: Ice3a6e830f0d3f759f8edaa3e5d3cbd39c69ce8d