Adjusts the USAP pool refill mechanism.

This patch adjusts the USAP pool refill mechanism so that it is less
disruptive to launching applications.  Refill events are now delayed
by several (3) seconds if delaying wouldn't cause the pool to be below
the minimum population size.

This patch also adjusts the process priorities for the USAPs and other
processes spawned from the Zygote so that they behave better in the
context they are spawned in.

Bug: 131362095
Test: booted & launched apps & checked logs
Change-Id: If26d625c4b1e5e8eee54dcdacb32360b0d852829