StringBuffer cleanup

This patch cleans up StringBuffer use, mostly replacing StringBuffer
with StringBuilder or plain String concatenation for short sequences
(or constants).

* HexEncode: additionally, avoid using a separate return variable,
  drop the useless ":" handling in bytesToHexString(), and add the
  test class from l2switch.
* HardcodedModuleFactoriesResolver: rework the conflicting module
  handling.

Change-Id: Id76e91bba9ce40bd8ed5947c2d40e3a7baf0a949
Signed-off-by: Stephen Kitt <skitt@redhat.com>