Use Constants.OBJECT_ID_STRING_LENGTH instead of LEN * 2

A few locations were doing OBJECT_ID_LENGTH * 2 on their own, as
the old STR_LEN constant wasn't visible.  Replace them with the
new public constant OBJECT_ID_STRING_LENGTH.

Change-Id: Id39bddb52de8c65bb097de042e9d4ed99598201f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>