[monodroid] Introduce EmbeddedAssemblies class (#2606)

As part of our continued "C++ification" effort (6c4939d, etc.),
introduce a new `EmbeddedAssemblies` which handles `.dll` loading.

Remove a number of `MONO_API` functions such as
`monodroid_embedded_assemblies_register_from()`, as they don't appear
to be used anywhere.  (Some were intended to be used by Mono Unreal
Engine, but we're not sure if that's still the case.)

We can restore them if needed.

Note: `monodroid_embedded_assemblies_set_assemblies_prefix()` is
used externally by [Embeddinator-4000][0], and is preserved.

Additionally, remove many `printf()` family calls for string
generation, and use **strcat**(3) or **strdup**(3) instead, as
`strcat()`/`strdup()` can be faster than `snprintf()` or
`monodroid_strdup_printf()`.

[0]: https://github.com/mono/Embeddinator-4000/blob/c237e21b63df40799ef251350ee961846c549553/support/mono_embeddinator.c#L62

Author: grendello