Revert "Adjusts the USAP pool refill mechanism."

This reverts commit 0779af62.

Reason for revert: Breaks cts SignedConfigHostTest

Note: This CL effectively reverts commit:
54b8d7ea
"Make the USAP Pool refill delay configurable."

As that builds on top of the original CL being reverted. Only the
ZygoteServer portion of that change is reverted.

Test: atest SignedConfigHostTest
(on an acloud instance)
Bug: 132794985
Merged-In: I1327a22d103452bed46d191ec2bafb72a5e095ca
Change-Id: I1327a22d103452bed46d191ec2bafb72a5e095ca