We have some amount of inconsistency around enumeration returns. Improve
the situation by making forName() return a nullable value and add
ofName()/ofValue() methods which throw IAE.
While we are modifying the template, also add proper @Nullable and
@NonNull annotations.
JIRA: MDSAL-754
Change-Id: I07fe4b408174ba830bcc1369ed70597ea2538a85
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>