[SysUI][Floaty] Add a flow to detect change in active user or assistant

* Add a flow which combines active user flow and active assistant flow
* Cache active user and assistant to compare it against the saved
  preference later to check they were set for which user and assistant

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

Change-Id: Ie8c8f20df8a3650617aa2b1e7c65cb6ce7b3e18f