ash: Remove old lock animation implementation.

This deletes the SessionStateControllerImpl class (formerly
used when the --ash-disable-new-lock-animations flag, which
I'm also removing, was set) and moves
LockStateControllerImpl2 into LockStateController (which was
formerly an interface).

BUG=294310

Review URL: https://codereview.chromium.org/24980006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226298 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226298