Introduce target dirty segment ratio tunable parameter

We introduce a new parameter of target dirty segment ratio,
which can be used to set a target dirty / (dirty + free) segments
ratio. For example, if we set this as 80%, GC sleep time will be
calculated to achieve this ratio in a GC period.

Test: check smart idle maint log of StorageManagerService
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: I73f2bcf4bdb810164c174bd0d2518b15d577d5d5