Fix part of Mono.Posix build issue regarding MonoDroidSDK discovery.

[atsushieno]

We can build xamarin-android on Linux only because we somehow had a
working monodroid setup with MONO_ANDROID_PATH. Hence, fresh build hits:

bin/Debug/lib/xbuild/Xamarin/Android/Xamarin.Android.Common.targets:
 error : Error executing task ResolveSdks: Value cannot be null.

Now that Mono.Posix and some other libs depends on existing SDK,
MonoDroidSdkUnix needs to also deal with bootstrap build.

So far, the SDK sanity check uses generator.exe which does not exist when
we are building Mono.Posix, so use class-parse.exe which exists instead.

Also we only build 6.0 and UseLatestSDK somehow tries to find 6.0.99 and
fails, so use 6.0 instead.

[dellis1972]

@atsushieno would it not be better to make use of the "SearchPaths" property? its resolved later in the FindRuntime method.

[atsushieno]

SearchPaths need to be absolute paths, which do not fit with this not-fixed path. Or do you mean something else?

[dellis1972]

My point was this feels like a hack. We could instead add the relative path to the "additionalSearchPaths" variable [1] which would be more in keeping of how this method works.

[1] https://github.com/atsushieno/xamarin-android/blob/8db545653004741711f39d75fa598e1f5a88ace6/src/Xamarin.Android.Build.Utilities/Sdks/MonoDroidSdkUnix.cs#L40

[atsushieno]

I don't think that is actually better, because in xabuild script we set MONO_ANDROID_PATH and in that case my code will never hit that workaround path, while your suggestion will.

[dellis1972]

if MONO_ANDROID_PATH is a valid value (via the xabuild script) the code will exit [1] so in that case your code path will not be hit and neither would mine. We already have code in place to search alternate directories, I'm just suggesting we use it :) e.g

var additionalSearchPaths = new [] {
  Path.GetFullPath (Path.Combine (new Uri (GetType ().Assembly.CodeBase).LocalPath, "..", "..", "..", "..", "..", "lib", "mandroid")),
  Path.Combine (personal, @".xamarin.android/lib/mandroid"),
};

[1] https://github.com/atsushieno/xamarin-android/blob/8db545653004741711f39d75fa598e1f5a88ace6/src/Xamarin.Android.Build.Utilities/Sdks/MonoDroidSdkUnix.cs#L32

[atsushieno]

Alright, I was confused about the formal(?) behaviour of the path probe, thought that user home thing is not kind of hack around some issue.

[grendello]

LGTM