Use new AssistUtils(Base) to override SysUI Assist invocations.

AssistUtils#getSysUiAssistOverrideInvocationTypes(): Sent
over SysUiProxy to request overriding these invocation types.

AssistUtils#tryStartAssistOverride(): Called by OverviewProxy
(TouchInteractionService) for previously requested overrides.
Also used within Launcher for Taskbar and QSB to override
other invocation logic. May return false to indicate that
the override was not handled, so a fallback may be desired.

Bug: 295874732
Test: Manual
Change-Id: I488f3b7da1feb2663feab5d04dfa8d605c070efb