cros: Fix VK not closing when KSV closes

- Make KeyboardController update its observed IME when it is
  enabled/activated/shown. There could be multiple IME instances
  with mojo apps. This allows KeyboardController to follow
  the active IME;
- Change ChromeKeyboardUI to return the active IME from IMEBridge
  instead of the globally shared IME;

Bug: 876589
Change-Id: Ieb4cde7131969da3c9182d6bd5447dbe604440fd
Reviewed-on: https://chromium-review.googlesource.com/1194986
Reviewed-by: Shu Chen <shuchen@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588653}