Reviewing our codebase, we found a place where the `BuildApk` task was
calling an overload for `MonoAndroidHelper.IsReferenceAssembly`.
This presented two problems:
1. It was using the `InMemory` option
2. We were not calling `Dispose()`!
Case No.1 was bad, since we basically load every assembly into memory!
    foreach (ITaskItem assembly in ResolvedUserAssemblies) {
        if (MonoAndroidHelper.IsReferenceAssembly (assembly.ItemSpec)) {
    //...
    foreach (ITaskItem assembly in ResolvedFrameworkAssemblies) {
        if (MonoAndroidHelper.IsReferenceAssembly (assembly.ItemSpec)) {
Likewise, we need to call `Dispose` here, since a lock could be held
on the file here on Windows.
The changes here made `BuildApk` slower (and more correct!), but the
overall build time better.
Before:
    5890 ms  BuildApk                                   1 calls
    Time Elapsed 00:01:02.59
After:
    6377 ms  BuildApk                                   1 calls
    Time Elapsed 00:00:59.37
I think the `AssemblyDefinition` instances must have been getting
cleaned up later in the build by the GC (if we were lucky), and also
used *a lot* more memory. This would contribute to overall build time.