[SysUI][Floaty] Provide no-op CoreStartable when feature disabled

Bug: 399263897
Test: Manually via logging, observed no-op CoreStartable was provide when
feature was disabled
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect

Change-Id: I7f03c201cfb47d338f500c9dfec6dfe5935f2c6b