desktop-pwas: Move uninstall placeholder logic into InstallationTask

BookmarkAppInstallationTask is in charge of performing all necessary
steps to install an app, so it should be responsible for uninstalling
the placeholder app before installing the new app.

Bug: 916381
Change-Id: I1aa65d80fb355bb1501a617eeced0e446645392a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609752
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660287}