Droptest: optimize macToString

This elides the creation of multiple objects and rather formats the
bytes quickly.

Change-Id: Iabac79bf86c0b2098bffe69f221797f2fd1c326f
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Optimize conversionMapping table

Once the table has been initialized, we can switch it to immutable, as
the keys (Short) provide fast hashCode/equals. Saves memory and
increases lookup speed.

Change-Id: I4698f44ce5a8e83c7c54e544e5ab72487f22a92e
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>