[runtime] Add a "mobile" runtime version.

Xamarin.Android has historically used the "v2.0.50727" runtime
version -- which works -- but has the side effect that Mono constantly
generates runtime "warnings" akin to:

	The request to load the assembly mscorlib v2.0.5.0 was remapped to v2.0.0.0

The reason for this is that all of the Xamarin.Android assemblies have
the version number v2.0.5.0, while the "v2.0.50727" runtime version
expects assemblies to have version number v2.0.0.0; they differ.

Result: Mono warns (appropriately), and I do my best to ignore the
~meaningless warning message. (For years and years and years...)

It's time we fixed this.

Introduce a "mobile" runtime version which specifies the
Xamarin.Android v2.0.5.0 assembly versions. Xamarin.Android 4.14+ will
then specify the "mobile" runtime version, "fixing" the assembly remap
messages.