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(1) A SearchResultRanker class with a simple AppLaunchPredictor is
    added.

(2) SearchResultRanker lives inside AppSearchProvider and re-rank
    the suggested apps.

(3) training signal is passed from AppListClientImpl to
    SearchController, then to all SearchProviders; but only
    AppSearchProvider responds to that training signal by passing the
    signal to SearchResultRanker.


Change-Id: I9879bcf1932e2928720c43680ca37edf35c3a785

Bug: 871674
Change-Id: I9879bcf1932e2928720c43680ca37edf35c3a785
Reviewed-on: https://chromium-review.googlesource.com/1164863
Commit-Queue: Charles . <charleszhao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583953}