Change ScanResult#isConnectable from nullable boolean to non-nullable enum

[mikolak]

#586 (comment)

Connectivity with 3 states:
Connectivity.noInfo (or notAvailable?), Connectivity.notConnectable, Connectivity.Connectable

[okocsis]

Right now it's Bool? isConnectable so ternarily [null, false, true]
it's nullable because it's only available on iOS host OS, on android it is still connectable so null very importantly does not mean false at all!
Therefore I'd also recommend using a less misleading representation being the
enum Connectivity [ .notAvailable| .unsupported, . notConnectable, .connectable]
correspondingly mapping to the Bool? values above.