Use InputEventAssigner to assign input to frame

When a frame is determined to be caused by an input event, it will be
labeled with a specific input event id. The newly added entity,
InputEventAssigner, is responsible for determining which input
event we should use.

For regular ACTION_MOVE events, we will take the latest input event as
the event that has caused the frame. For the initial gesture
(ACTION_DOWN), we will use the first event (the ACTION_DOWN itself) to
figure out the down latency. This will allow us to split up 'down' and
'scroll' latencies.

Bug: 169866723
Test: looked at the data printed locally to logcat
Test: atest InputTests
Change-Id: I8513a36960e5d652d07655d1267e758d0c59ced7