Reland "Disallow shrinking threadpool size once started."

This reverts commit 0a09c76c713ded94ab38e6917402251977923baf.

Bug: 147699567
Test: atest LibStatsPullTests now passes
Reason for revert: b/150904425 is fixed.

Change-Id: I00451b5f146dbb9086d395a8bf50e5929a6b7839
1 file changed