Add OverridableFlow for Spa

One typical use case is that when a switch state comes from a flow, and
the user toggle the switch to change the value, we can modify the flow
to show the latest state.

Also temporary copy the FlowExt from AndroidX lifecycle, to use the
collectAsStateWithLifecycle(), so we can show the latest data from flow
when each on start lifecycle event.

Bug: 236346018
Test: Unit test
Change-Id: Ib34ae9c5ce0b9122d11032aeed173033df5cc2d1