AAPT2: Fix compatible-screens element in AndroidManifest.xml

Fix an issue where the compatible-screens element was not being
populated correctly. The previous version was missing the screenSize
attribute which is mandatory. The attributes were also missing the
resource ID from the framework library as these are a part of the public
API.

Change-Id: I2f594c2259831dbbd96c58db4ba55e8288d4231e
Test: unit tests
Test: manually split an APK and dumped with aapt
diff --git a/tools/aapt2/optimize/MultiApkGenerator.h b/tools/aapt2/optimize/MultiApkGenerator.h
index 19f64cc..c858879 100644
--- a/tools/aapt2/optimize/MultiApkGenerator.h
+++ b/tools/aapt2/optimize/MultiApkGenerator.h
@@ -63,6 +63,11 @@
   bool UpdateManifest(const configuration::OutputArtifact& artifact,
                       std::unique_ptr<xml::XmlResource>* updated_manifest, IDiagnostics* diag);
 
+  /**
+   * Adds the <screen> elements to the parent node for the provided density configuration.
+   */
+  void AddScreens(const ConfigDescription& config, xml::Element* parent);
+
   LoadedApk* apk_;
   IAaptContext* context_;
 };