Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=61073
Context: a296784

When we added the new `classes.zip` functionality, there was one oddity
present. On the first build, everything appears to be normal. On the
second build, `classes.zip` appeared to contain itself inside itself???

To make things even stranger, I wrote a unit test that contains a
`Java.Lang.Object` subclass in which its type and namespace names change
on each write. This worked fine on macOS, even with the oddity of
`classes.zip` continually zipping itself inside itself... The new java
sources would appear at the bottom of the list within the zip file.

Everything broke down when I tried my unit test on Windows, the case of
`classes.zip` trying to compress itself inside itself was silently
failing. It was not processing any further files after the failing zip
entry, and so *in theory* this situation would cause many Java stubs to
be missing from the zip.

Solution? Let's just put `classes.zip` up one directory.

So instead of:
```
obj/Debug/android/bin/classes/classes.zip
```
Move it to:
```
obj/Debug/android/bin/classes.zip
```

This prevents the `classes` directory from containing `classes.zip`, and
our strange case of `classes.zip` inception.

Merry Christmas! I'm going to go play Skylanders now.