AAPT2: Add targetConfig to AndroidManifest.xml

Splits may include more/different configurations than they
were originally specified to include. This happens, for instance,
when a particular requested density doesn't exist. A fallback density
resource is chosen for the split.

This makes programmatic configuration targetting difficult, as the APK
may report that it has resources for multiple densities, while it was
meant to target only a specific density.

This change encodes a comma separated list of the intended configurations
for the split in an attribute called 'targetConfig' on the <manifest> tag of
AndroidManifest.xml.

Test: manual
Change-Id: I3167e9341a434abc34ed2ddb8f740762b40b1ab7

tools/aapt2/cmd/Util.cpp

@@ -179,6 +179,13 @@ std::unique_ptr<xml::XmlResource> GenerateSplitManifest(const AppInfo& app_info,
         xml::Attribute{"", "configForSplit", app_info.split_name.value()});
   }
 
+  // Splits may contain more configurations than originally desired (fallback densities, etc.).
+  // This makes programmatic discovery of split targetting difficult. Encode the original
+  // split constraints intended for this split.
+  std::stringstream target_config_str;
+  target_config_str << util::Joiner(constraints.configs, ",");
+  manifest_el->attributes.push_back(xml::Attribute{"", "targetConfig", target_config_str.str()});
+
   std::unique_ptr<xml::Element> application_el = util::make_unique<xml::Element>();
   application_el->name = "application";
   application_el->attributes.push_back(