Add IsValidLevel.

The range of level values will soon be
non continuous, so the check against
LAST_PLUS_ONE is no longer sufficient.
Change it to a proper predicate.

Also delete the unused LAST_PLUS_ONE after
this change.

NO_IFTTT=editing comments

Test: TH
Bug: 314845349
Change-Id: If67bbd20057b53b688f4f0a9a1f8210de4687f6e
2 files changed