iwa: Save the selected update channel in the db in IWA installer.

This CL adds the plumbing to persist the update channel selected by the
user during the IWA installation process. It does not contain UI changes
needed to enable user choice. If the user does not select an update
channel, the "default" channel is assumed as the chosen one.

Changes overview:
- Added UpdateIwaChannelInDatabase to update the app's IsolationData with the selected channel.
- Updated IsolatedWebAppInstallerView to allow the controller to retrieve the selected update channel and pass the list of available channels to the metadata screen.
- Relaxed the restriction in IsolationData::Builder::SetUpdateChannel
that previously only allowed update channels for dev-mode installs, as
this feature is now being expanded to user installed IWAs.

Change-Id: I63f9a19054ccc1eab9b01183454dc6b73e951e03
Bug: 500326341
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7796457
Reviewed-by: Simon Hangl <simonha@google.com>
Commit-Queue: Magdalena Skarbińska <mskarbinska@google.com>
Reviewed-by: Andrew Rayskiy <greengrape@google.com>
Cr-Commit-Position: refs/heads/main@{#1629905}