Consolidate tracking of touch state in Editor (editable TextView)

The goal of these changes is to consolidate the state that needs to be
tracked for user motion events (touches, clicks, etc), in order to make
it easier to add/update functionality in the future.

These changes are intended to fully preserve existing behavior from the
user perspective. For the most part, the logic is completely unchanged
(just that the code is pulled out), but it's worth calling out the
following subtle changes to the logic. These changes don't affect any
user-visible behavior.

1) In Editor.onTouchEvent, the recorded position of the last down event
is now updated before SelectionModifierCursorController.onTouchEvent()
is invoked.

2) In Editor.updateTapState (now in EditorTouchState.update), instead
of using SystemClock.uptimeMillis() for the event time, we use
event.getEventTime().

Bug: 143852764
Test: Manual testing and unit test
  atest FrameworksCoreTests:EditorTouchStateTest
Change-Id: Ic40b5e53412294d365bc9d787e7013a44fe11ffa