ash: Convert DetailedViewController::CreateView() to std::unique_ptr

The function constructs a view, so return std::unique_ptr<views::View>
for consistency with the new views APIs.

Fix some includes along the way.

Bug: b:251742508
Test: ash_unittests
Change-Id: I9e39e5168e39128b0b13b690d8ce1ed68784fead
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4029523


Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072313}