Enabled TLS 1.1 and 1.2 support on platforms >= 16 and <= 20

[grendello]

Context: #1615

It turns out that the older platforms do support TLS 1.1 and 1.2 protocols but
that they don't enable it by default. Thanks to code provided by
https://github.com/gameleon-dev in the issue linked to above we now support it
too.

Updated the TLS 1.2 tests to enable them on platforms >= 16

[jonpryor]

Thanks to code provided by https://github.com/gameleon-dev

Where/how was that code provided?

[jonpryor]

Why the empty line?

[jonpryor]

You should not do this.

The problem is "reference sharing": every AndroidClientHandler instance (on API-16..20 devices) will have an OldAndroidSSLSocketFactory instance -- so let's call it N instances of AndroidClientHandler & OldAndroidSSLSocketFactory -- each of which will refer to the same SSLSocketFactory value returned by SSLSocketFactory.Default.

This results in "unintentional spooky action at a distance", a'la:

var a = new AndroidClientHandler (...);
var b = new AndroidClientHandler (...);

a.Dispose(); /* ...or otherwise cause `a.httpsConnection.SSLSocketFactory.Dispose()` to be invoked */
/* use `b` */

Once a.httpsConnection.SSLSocketFactory has been disposed, b.httpsConnection.SSLSocketFactory can no longer be used either, because even though the httpsConnection.SSLSocketFactory instances differ, OldAndroidSSLSocketFactory.factory for all instances refers to the same instance, and when that instance was disposed from a, b can no longer use it.

[grendello]

Thanks to code provided by https://github.com/gameleon-dev

Where/how was that code provided?

See the commit message:

Thanks to code provided by
https://github.com/gameleon-dev in the **issue linked to above** we now support it
too.

[grendello]

build

[grendello]

build

[grendello]

build