[Floaty] Avoid collecting flow is flag is not enabled

Makes isInvocationEffectEnabledByAssistantFlow into a cold flow that is only started
when isSqueezeEffectEnabled is collected (which is flag guarded).

Bug: 405328848
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect
Test: Unit and Manual
Change-Id: I40aec13e1724df2a0cf3fde14889c5215a96654b