isAvailable() is a fake false condition

[guihsdev]

we have 3 conditions on nfcAdapter

    when {
                    this.nfcAdapter == null -> result.success("not_supported")
                    nfcAdapter.isEnabled -> result.success("available")
                    else -> result.success("disabled")
                }

this is for considering when a device don't have NFC support, when NFC is disabled and when is activated.
On simple use of IsAvailable() we could just receive false when has NFC but is only disabled.

i know iOS works on the way IsAvailable works today, but i think a platform specific case need to be implemented...

[okadan]

This issue will be resolved in v4.0.0. (#128)