Sets invocation effect enabled to false by default

This was always the plan, but we kept it enabled by default to enable testing
before Gemini implemented calling the API.

Bug: 415708075
Test: Unit and Manual
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect
Change-Id: Ie259e0499b7b3a095e6bebf2327af326d50506c9