Talked with zzz and did more research, the overhead of Enum is minimal compared
to the benefits it provides.

Enums often require more than twice as much memory as static constants, and
should be strictly avoided on Android.
https://developer.android.com/training/articles/memory.html#Overhead

The advantage of this change is that client library users can directly compare
the status values they get from IRouterState to the constants, instead of
parsing a string representation of an Enum.

If we move to increment-by-one instead of calculating the version code, we need
to keep the code above the current highest generated code, so that F-Droid will
order new builds correctly above old builds.

If we decide to go back to calculating version codes later, it is possible - we
can increment the version code 2047 times before it will clash with the one for
0.9.14.

now on Java 6 (API 9).