[Xamarin.Android.Build.Tasks] fix for <Aapt/>

[jonathanpeppers]

Context: http://build.devdiv.io/2527883

Our integration build for the Xamarin designer has caught a bug in the
following situation:

• macOS-only (Windows is OK)
• $(AndroidUseAapt2) is False
• Using Xamarin.Android.Support.CustomTabs version 23.x

I was able to reproduce the problem by changing the
BuildTest.ResourceExtraction test.

The error message is:

/Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Common.targets(1765,2):
    error APT0000: resource directory '/private/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/ADT-0/RT-MyDriving-1/MyDriving.Android/obj/Debug/resourcecache/3288F845948C6081AB114E82B328E204/res' does not exist

The reason this fails is that this particular library does not have a
res directory:

$ ls -l ~/.local/share/Xamarin/Xamarin.Android.Support.CustomTabs/23.1.1.0/embedded
total 64
 8 -rw-r--r--  1 jonathanpeppers  staff    861 Mar 26 11:18 AndroidManifest.xml
 0 drwxr-xr-x  3 jonathanpeppers  staff     96 Mar 26 11:18 aapt
56 -rw-r--r--  1 jonathanpeppers  staff  24743 Mar 26 11:18 classes.jar

The fix is to add a Directory.Exists check, this seems to fix the
unit test and the failing project used by the designer build.

[jonpryor]

Do we really need to repeat this much code, changing only the ITaskItem[] source?

For example, with C# local functions:

void AddResourceDirectories(ITaskItem[] directories) {
    if (directories == null) {
        return;
    }
    foreach (var dir in directories) {
        if (!Directory.Exists (dir.ItemSpec))
            continue;
        var resdir = dir.ItemSpec.TrimEnd ('\\');
        cmd.AppendSwitchIfNotNull ("-S ", resdir);
    }
}
AddResourceDirectories (AdditionalResourceDirectories);
AddResourceDirectories (AdditionalAndroidResourcePaths);

[jonathanpeppers]

One does a Path.Combine (dir, "res") and the other does TrimEnd().

I could use a local function, here but I'd also have to do something like:

void AddResourceDirectories(ITaskItem[] directories, Func<string, string> func) {
    if (directories == null) {
        return;
    }
    foreach (var dir in directories) {
        if (!Directory.Exists (dir.ItemSpec))
            continue;
        var resdir = func (dir.ItemSpec);
        cmd.AppendSwitchIfNotNull ("-S ", resdir);
    }
}
AddResourceDirectories (AdditionalResourceDirectories, d => d.TrimEnd ('\\'));
AddResourceDirectories (AdditionalAndroidResourcePaths, d => Path.Combine (d, "res"));

At that point, not sure if it's simpler?