wallpaper: rename is_locked to is_session_active_

is_session_active is clearer about what the variable means.
is_locked was misleading because it was true for many states
where the device is not actually locked, like OOBE and
LOGGED_IN_NOT_ACTIVE.

Bug: none
Test: ash_unittests --gtest_filter=*Wallpaper*
Change-Id: Ibc87b93f0db892caca467d1805cef737d000d7a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4456902
Commit-Queue: Jeffrey Young <cowmoo@google.com>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Erica Lee <ericamlee@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1136071}