[SysUI][Floaty] Use bug-fix flag for assistant invocation effect

* Replaces enable_lpp_squeeze_effect flag with new bug-fix flag enable_lpp_assist_invocation_effect

Bug: 399263897
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect
Test: EXEMPT no new tests added
Change-Id: I0bf0a7f01a41e6dffcd30b3bdd5b6f3fb0fd006e