[Read Anything] Define default theme from system theme

In this CL, we define the default theme for Read Anything from the
system theme. If the user is on dark mode, the default theme option for
Read Anything will be the Read Anything dark theme. For light mode, the
default theme will be the Read Anything light theme.

This CL also moves the creation of the Read Anything theme object from
the model to the page handler. The model initializes the ColorIds for
the themes and those are passed around as needed. When we reach the page
handler, we will have access to the color provider (since its a View
implementation) to convert the theme ColorIds to SKColors to make the
theme object.

Bug: 1266555
Change-Id: Ia4a51b056a5ac3ead1a8a8395eced8ea047f5f01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3987769


Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: Jocelyn Tran <jocelyntran@google.com>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072314}