Move STAT_TYPE enum to source file.

In the header, all we need is number of stat types, not the names for actual
types.

Removing it avoids names like 'Y', 'U', 'V' and 'ALL' being visible
in all files that include the encoder.h header.

Change-Id: I874a73a3cfe6bcb29aedea102077a52addc49af6