Fix a bug in the USAP Pool refill logic.

This CL addresses a bug in the USAP Pool refill logic that could cause
refill events to be delayed indefinitely.  This would not prevent future
refill events from triggering, but could cause temporary degradation in
USAP performance.

Test: launch applications from USAP pool and inspect log
Bug: 177795408
Merged-In: I2e46eae431a1d9671d31c29b3552cd0287c66239
Change-Id: I2e46eae431a1d9671d31c29b3552cd0287c66239