ash: Rename SessionController to SessionControllerImpl

No functionality change. Just renaming so that SessionContrller/Client
could be used to replace mojom::SessionContrller/Client.

Bug: 958214
Change-Id: I7fb597f955073564fb85c8a55e1c3aacc8cc4fe4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597201
Reviewed-by: Scott Violet <[email protected]>
Commit-Queue: Xiyuan Xia <[email protected]>
Cr-Commit-Position: refs/heads/master@{#657021}
218 files changed