Declare AdaptyErrorCode static variables as constant

[jezsung]

The AdaptyErrorCode variables are currently declared as static final.

This makes it impossible to use the error codes with the switch expression.

I suggest the following code changes to all static variables:

class AdaptyErrorCode {
  static final int unknown = 0;
  // to
  static const int unknown = 0;
...

[x401om]

Hi @jezsung! That seems to be a good enhancement. I think we will implement that in the next release. Thank you!