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This adds a class that holds push-install event logs for all apps.
The class is not hooked up to anything yet, so no log entries are
read or written.

Bug: b/73277923
Test: unit_tests
Change-Id: Ic95c02d9a47b56492e9ac9ef9f0d8e1ce8aa4ea1
Reviewed-on: https://chromium-review.googlesource.com/916243
Commit-Queue: Bartosz Fabianowski <bartfab@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537676}