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.

(cherry picked from commit e82ecc6bb4235e792b21964019ea07fc62e9ad3c)
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Change-Id: I21c0b1660f723f2456f36bd4af1ac203a555400a