The bionic benchmarks set the decay time in various ways, but
don't necessarily restore it properly. Add a new method for
getting the current decay time and then a way to restore it.
Right now the assumption is that the decay time defaults to zero,
but in the near future that assumption might be incorrect. Therefore
using this method will future proof the code.
Bug: 302212507
Test: Unit tests pass for both static and dynamic executables.
Test: Ran bionic benchmarks that were modified.
Change-Id: Ia77ff9ffee3081c5c1c02cb4309880f33b284e82
