Review reflection usage in src/Mono.Android.Export/CallbackCode.cs

[radekdoulik]

We are preserving many types unconditinally in Mono.Android.xml because of reflection used here.

It should be possible to remove some of the types from the list as illink is smarter about reflection and hopefully also update some of the code to be more linker friendly.

Also use the new dependency attributes when possible.

[jonpryor]

Pie-in-the-sky alternative: if jnimarshalmethod-gen ever becomes A Thing, then we can type-forward Java.Interop.ExportAttribute, Mono.Android to Java.Interop.ExportAttribute, Java.Interop and have build-time marshal method generation for [Export] methods.