Move startup thread pool back into runtime

Added logic in heap trim to delete the thread pool, if there are no
active users.

Added a scoped accessor to prevent ref counting errors.

Motivation, have workers already created when the app images are
loaded.

Bug: 116052292
Test: test-art-host
Change-Id: I8ea776d74e88601222a9989e0c6dac34cf77c683