[stash] append _VAL to ValueType enum options

Append _VAL to the options in the ValueType enum, as the INT option was
causing a name collision in Dart, and it's best to try to avoid
identifiers that can cause name collisions in the first place.

CF-279 #done

TEST: `fx build` passes, tryjobs
Change-Id: I8f4cd32e8fe9c92a811db88fbb0b661929317fbf
2 files changed