vold: do not set gc sleep time with zero dirty segments

When there is not enough dirty segments to reclaim, we don't have to set
gc sleep time.

Test: Run smart idle maint with zero dirty segments
Bug: 202283480
Bug: 181079477
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: If6a9965ecb9b1a91bf5d0dd763d5fa0af63348e0