ShellStream: Add ReadAsync implementation

[qmfrederik]

We encourage asynchronously executing a command on the device using AdbClient.ExecuteRemoteCommandAsync.

However, this will then use a ShellStream which only supports synchronous reading data off the stream.

Additionally, to support cancellation, AdbClient.ExecuteRemoteCommandAsync will invoke cancellationToken.Register(() => socket.Dispose()).

However, on Unix, calling socket.Dispose will not unblock synchronous calls for current versions of .NET Core. (This is fixed in .NET 5.0). dotnet/corefx#38804 has the details.

As a result, cancellation of ExecuteRemoteCommandAsync methods is effectively impossible.

This PR implements aynchronous methods in ShellStream, making the scenario work.