Implement persistence of assistant configuration for Floaty

Only persists the preferences for a single user + assistant combination.
The goal is only to avoid issues like SysUi crashes resetting the configuration.

Bug: 405328848
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect
Test: Unit and Manual
Change-Id: Idcab788fdb2de6cdbb83b29c34e1fea6c1019417