[SysUI][Floaty] Extract preferences logic to a separate class

* Introduce InvocationEffectPreferences which contains SharedPreferences
  related data for assistant invocation effect

Bug: 412616963
Test: NONE just introducing a class, tests will be added in later CLs
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect

Change-Id: I3125f29d269a33c7b3e69781ffe2f999ff965cd8