Create a LegacyTransitions to manage combined WCT + remoteanimation

Use this to implement Drag-to-split transition

BYPASS_INCLUSIVE_LANGUAGE_REASON=using existing API.

Bug: 192291727
Test: drag from taskbar into split and observe. Also,
      drag same-app into split and observe no-op.
Change-Id: I710805c85c7d57ae8eebe18e5df7fca899f1b882