Navigation tabs have all been replaced with spinners, because there are no nice
tab libraries for API 9+.

Source: https://gist.github.com/str4d/22cac7a3f70bc227cdca
License: http://opensource.org/licenses/MIT

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.