[SysUI][Floaty] Create flow to determine if effect enabled by assistant

* Move "isInvocationEffectEnabledByAssistant" to the preferences utility
  class
* In later CLs entire info of assistant preferences will be abstracted
  out in InvocationEffectPreferences to act as single source of truth

Bug: 412616963
Test: NONE just introducing a flow, it's not consumed anywhere
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect

Change-Id: I6315bb8c49049da094549db164bf48d3e1372a87