Fix dark mode in feed v2 with isolated splits enabled

Dark mode was not working correctly since layouts were being inflated
with the feed isolated split context and not the activity context. This
change pulls only the ClassLoader from the feed isolated split context,
and adds a util which can be used elsewhere.

Bug: 1126301
Change-Id: I9f1da598cb1df2a33c1095bc9afb8e451b518991
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531516
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826359}