vold: fix the range of stopped state of idleMaint

When it is stuck in runDevGC() for an unexpected reason, now it cannot
prevent the other callers from entering into it again. Fix it.

Bug: 232297944
Test: run "sm idle-maint run" twice & check whether they are overlapped
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: I785c8aeebd8fcf58c34d9be9968d99634d0b420a