DPWA: dont check AreWebAppsUserInstallable for non-user install

`InstallWebAppOnManifestValidated` is used by both user installs and
externally managed installs. The externally managed installs happen
in the background and shouldn't check for
`AreWebAppsUserInstallable`.

To detangle the control flow, follow up work will be done to
denormalize the install logic to individual install commands
instead of sharing the code paths in `WebAppInstallTask`.

Bug: 1335280
Change-Id: Iea1b080d69be7a6b88212fa0a7586e88766c6e8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3699117
Reviewed-by: Dibyajyoti Pal <dibyapal@chromium.org>
Commit-Queue: Phillis Tang <phillis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1013114}