Run "plugins" singleton in parallel with the rest.
This completes the changes in aosp/q/topic:"parallel-singletons".
(cherry picked from commit 30bf7fe32d0ba644cd181645fab3c714a9a11b2f)
Bug: 281536768
Test: manual, presubmits
Change-Id: I6529e957fbcca6c18441f823a00061db7556e303