Get rid of a duplicate constant for SHA-1 length

Since Constants.OBJECT_ID_LENGTH is a compile time constant we
can be sure that it will always be inlined. The same goes for the
associated constant STR_LEN which is now refactored to the Constant
class and given a name better suited for wider use.

Change-Id: I03f52131e64edcd0aa74bbbf36e7d42faaf4a698
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>