sunfish/scanner: Observe pinned state from feature watcher

https://crrev.com/c/6337094 disables Sunfish from being shown if there
is a pinned window.

This CL moves the explicit `UpdateFeatureStates()` call in
`CaptureModeController::OnPinnedStateChanged` to
`SunfishScannerFeatureWatcher`, making it explicit that the feature
watcher is updated when the pinned state changes.

This will be even more useful once Scanner also adds a pinned state
check in the following CLs.

Bug: b:403423199
Change-Id: I6a6a210064bce4b9002ed1a4f55d2aeb2d9c05b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6365375
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Michael Cui <mlcui@google.com>
Cr-Commit-Position: refs/heads/main@{#1435195}