Add ART module to extension SDKs

This is the only S module at the moment. Add a unit test to
derive_sdk_test to verify the multi-dessert logic works correctly.

Update the README with up-to-date instructions for adding new SDK
extensions.

Bug: 173188089
Test: atest derive_sdk_test
Change-Id: Id4fbbe13746d99d257342ff5d0dd17e6462d5336
diff --git a/derive_sdk/derive_sdk.cpp b/derive_sdk/derive_sdk.cpp
index 98b84a4..d87dfee 100644
--- a/derive_sdk/derive_sdk.cpp
+++ b/derive_sdk/derive_sdk.cpp
@@ -35,6 +35,7 @@
 namespace derivesdk {
 
 static const std::unordered_map<std::string, SdkModule> kApexNameToModule = {
+    {"com.android.art", SdkModule::ART},
     {"com.android.ipsec", SdkModule::IPSEC},
     {"com.android.media", SdkModule::MEDIA},
     {"com.android.mediaprovider", SdkModule::MEDIA_PROVIDER},
@@ -51,7 +52,7 @@
     SdkModule::TETHERING,
 };
 
-static const std::unordered_set<SdkModule> kSModules = {};
+static const std::unordered_set<SdkModule> kSModules = {SdkModule::ART};
 
 bool ReadDatabase(const std::string& db_path, ExtensionDatabase& db) {
   std::string contents;