CrOS Shelf: Get rid of 'split view' mode for shelf background

In the new UI, "maximized" and "split view" are treated the same in
specs, so there is no more need for a separate "split view" mode. This
folds it into the "maximized" mode.

Note that the only thing that _seems_ different in
shelf_background_animator is ShelfBackgroundAnimator::kMaxAlpha (255)
vs kShelfTranslucentMaximizedWindow (254), which should be virtually
impossible to distinguish.

This CL therefore does not have any visual effect (and doesn't
directly fix the linked bug, but is relevant).

Bug: 899289
Change-Id: I60947338176ac15ca016b1ba4edf13d16362cb24
Reviewed-on: https://chromium-review.googlesource.com/c/1469741
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631752}
bool ShelfLayoutManager::IsKeyboardShown() const {
  return !keyboard_displaced_bounds_.IsEmpty();
}

bool ShelfLayoutManager::IsKeyboardShown() const {
  return !keyboard_displaced_bounds_.IsEmpty();
}
