AAPT2: Support CtsContentTestCases build

- Add <feature-group> to ManifestFixer.
- Support <meta-data> in <instrumentation>
- Add support for <bag> and type="configVarying". Some CTS tests use this
  old notation, we need to support it (even though configVarying isn't
  anything supported by the framework convention).

Change-Id: I6946fa633ce513ea8437c1496db883cf27dcf6de
Test: make aapt2_tests
diff --git a/tools/aapt2/Resource.h b/tools/aapt2/Resource.h
index 1950ea3..4d915d9 100644
--- a/tools/aapt2/Resource.h
+++ b/tools/aapt2/Resource.h
@@ -44,6 +44,11 @@
   kAttrPrivate,
   kBool,
   kColor,
+
+  // Not really a type, but it shows up in some CTS tests and
+  // we need to continue respecting it.
+  kConfigVarying,
+
   kDimen,
   kDrawable,
   kFont,