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Changes:
Design doc: go/arc-app-shortcuts

Launch app shortcut has to be called by Android's LauncherApps'
startShortcut API. Thus provide a mojo API to do the work.

Bug: 803291
Test: tested on device with ag/3997639
Change-Id: Idece1c44836a9e43932b700b81d128883c15666d
Reviewed-on: https://chromium-review.googlesource.com/1041574
Commit-Queue: Qiang Xu <warx@google.com>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556106}