Support muxed accounts by default.

[tamirms]

Previously, in #348 we added opt in support for muxed.
But, by default, we rendered muxed accounts in their non muxed encoding.
We are now changing the default behavior so that muxed accounts are rendered using ther 'M' address encoding.

Fixes #397

[sreuland]

looks good, really enabled by the groundwork done earlier too. Just to confirm understanding, there's no way in sdk usage to disable Muxed account encoding right, the 'switch' here is hard-coded.

[edit], if configurable opt-out for muxed was needed, AccountConverter pretty much has it, minor tweaks to treat it like a jvm singleton would probably suffice:

public AccountConverter {
  private static boolean gEnableMuxed = true;
  private static AccountConverter instance;

  public static AccountConverter getInstance() {
     if (instance == null) {
         synchronized (AccountConverter.class) {
             if (instance == null) {
                 instance = new AccountConverter(gEnableMuxed);
              }
         }
     }
     return instance;
  }

  public static disableMuxed() {
     gEnabledMuxed = false;
  }

  ... remove enabledMuxed(), disableMuxed()

}

replace invocations of enabledMuxed() and disabledMuxed with getInstance(), SDK users would need to optionally call AccountConverter.dsableMuxed() once before invoking anything else in the SDK to disable, i.e. 'opt out' of muxed account encodings from that point on in the jvm.