Special interval value to always run benchmarks.

When interval is set of 0, always run benchmarks after every fstrim
pass, which is roughly daily.  We don't actually track the last
benchmark time for internal storage, so this is the best way we have
to enable this.

Also disable benchmarking completely when interval is set to -1.

Bug: 23942769
Change-Id: I11edfda063aac6e927c6cce56567169ebe83cb71