Add edge extension for ActivityEmbedding Shell transition animation

Refactor the edge extension logic from DefaultTransitionHandler to the
helper class so that we can reuse it for ActivityEmbedding.

Bug: 207070762
Test: manually verify with Settings.
Merged-In: I6a89b549e7b117caae75ceba6df8f41f5c4d4a15
Change-Id: I6a89b549e7b117caae75ceba6df8f41f5c4d4a15
8 files changed