One of the goals behind the Mono SDK/archive was to enable product builds
without having the entire Mono source present which means it should be possible
to remove Mono git submodule and build entire Xamarin.Android without any
issues. At the same time, we want to keep the ability to build Mono from source,
if necessary, for situations when a developer works on the BCL or the runtime
itself and there's no appropriate binary archive present. To that effect, this
commit implements the following:

  * Remove Mono submodule
  * Put Mono repo reference to the `.external` file
  * Make distinction between "commercial" and "regular" bits in the `.external`
    file so that we don't attempt to check out everything mentioned in there in
    situations when a developer doesn't have access to the commercial bits.
  * Use Mono linker sources from the archive
  * Remove `zlib-helper.c` from the `libmonodroid` build - the API implemented
    in there is no longer used by Mono.
  * `Xamarin.Android.Cecil.dll` and `Xamarin.Android.Cecil.Mdb.dll` are no
    longer built from source. Instead we use `Cecil` to reshape `Mono.Cecil` and
    conjure the two renamed and re-signed assemblies.
  * All the code which used `Xamarin.Android.Cecil` project reference now simply
    references the conjured asseblies (which are also added to the bundle)
  * Third Party Notices are generated from licenses found in the Mono archive.
  * Windows build downloads also the Darwin archive - in order to be able to use
    Mono Linker sources as well as licenses contained in this archive.