Improved timekeeping logic for USAP Pool refill mechanism.

This patch makes two changes to increase the reliability of the USAP
pool refill mechanism.  The first is to change the storage type of the
result of an expression from int to long.  This will help avoid
potential overflow issues.

The second change mitigates the impact of the non-monotonic system clock
having its value changed.  If the clock goes backwards in time past when
the initial event triggered the refill delay, the delay will now be reset.

Bug: 146614089
Test: Treehugger
Change-Id: Ic6f65de51757803b5ba95975bab3b9322a91c837
Merged-In: Ic6f65de51757803b5ba95975bab3b9322a91c837