Make smart idle maint's period changable

We need to make the period of smart idle maintenance service changable,
since we have to fine tune it per model.

Test: adb shell device_config put storage_native_boot smart_idle_maint_period 10
Bug: 202283480
Bug: 181079477
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: I0e9ef79fabefb1607cd5a9396a19ee7b4994a5ec