AAPT2: Implement density stripping and initial Split support

When a preferred density is supplied, the closest matching densities
will be selected, the rest stripped from the APK.

Split support will be enabled in a later CL. Command line support is still
needed, but the foundation is ready.

Bug:25958912
Change-Id: I56d599806b4ec4ffa24e17aad48d47130ca05c08
diff --git a/tools/aapt2/test/Context.h b/tools/aapt2/test/Context.h
index 555a539..e540cd7 100644
--- a/tools/aapt2/test/Context.h
+++ b/tools/aapt2/test/Context.h
@@ -71,6 +71,10 @@
         assert(mNameMangler && "test name mangler not set");
         return mNameMangler.get();
     }
+
+    bool verbose() override {
+        return false;
+    }
 };
 
 class ContextBuilder {