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This cl adds the mechanism to make Keyboard Shortcut Viewer (KSV)
searchable in launcher.
1. Adds an InternalDataSource.
2. Adds an InternalAppResult.
3. Adds unit tests for searching internal app.

Bug: 790748
Test: manual and AppSearchProviderTest.FetchInternalApp
Change-Id: I8cae318b51d75516575c9b02cb7565b1dc748951
Reviewed-on: https://chromium-review.googlesource.com/982995
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547054}