[Home] Polish bottom sheet content transitions

This change adds animation when transitioning between bottom sheet
content. The existing view is faded out completely before the new
view is faded in. The animation is applied to both the sheet content
and the toolbar view.

Notably, the onContentChanged() event is now called when the
transition animation has finished instead of immediately when
showContent(...) is called.

BUG=702717

Review-Url: https://codereview.chromium.org/2839673002
Cr-Commit-Position: refs/heads/master@{#466780}
2 files changed