[incfs] Fix the mount state callbacks processing

- use a never-existing storage ID as a job key
- order the jobs in the map to not skip them on changes, or,
  worse, never hang in a loop
- clear the local callbacks vector before moving to the next
  storage ID
- try to resume from the closest place on the next processing
  iteration

Bug: 183435580
Test: atest service.incremental_test
Change-Id: I36cd5d30c656bed62c20bd7a7f84fb58046a0933