Android Studio 3 and mvvmcross

Hi,
when a project layout file (xml) contains custom tags originating from an android library referenced in the android project (like mvvmcross), the gradle build fails as it can't find them.

It can't find them because XamarIdea does not add in ``app\build.gradle`` (in ``sourceSets/main/res.srcDirs`` tag) the ``obj/Debug/__library_projects__/*`` folders, which contains Android resources from these referenced assemblies.

A workaround is to manually update the build.gradle file and manually add each ``__library_projects__`` folder

```
apply plugin: 'com.android.application'

android {
compileSdkVersion 26
buildToolsVersion "26.0.2"
defaultConfig {
applicationId "com.egorbo.javashallowcopyforxamarinproject"
minSdkVersion 21
targetSdkVersion 26
versionCode 1
versionName "1.0"
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
}
}

sourceSets {
main {
res.srcDirs = [
'C:/Dev/repos/totoapp/Toto/Toto.Droid/Resources',
'C:/Dev/repos/totoapp/Toto/Toto.Droid/obj/Debug/__library_projects__/XamSvg.Droid/library_project_imports/res',
'C:/Dev/repos/totoapp/Toto/Toto.Droid/obj/Debug/__library_projects__/Vapolia.WheelPickerDroid/library_project_imports/res',
'C:/Dev/repos/totoapp/Toto/Toto.Droid/obj/Debug/__library_projects__/MvvmCross.Binding.Droid/library_project_imports/res',
'C:/Dev/repos/totoapp/Toto/Toto.Droid/obj/Debug/__library_projects__/MvvmCross.Droid.Support.V7.RecyclerView/library_project_imports/res'
]
}
}
}

```

Reproduce the failure with an Android project using custom XML tags from a referenced library, then inspect how app\build.gradle and sourceSets/main/res.srcDirs are generated. Done means the generated Gradle resource paths include the referenced obj/Debug/__library_projects__ folders and the project builds successfully in Android Studio.

Written by the indexing model from the issue text.