AAPT2: Embed version of framework an app was compiled against.

This change injects the 'android:compileSdkVersion' and
'android:compileSdkVersionCodename' into an app's AndroidManifest.xml,
whose values are dervied from the version name and version code of the
framework AndroidManifest.xml that is being linked against.

Bug: 63388434
Test: make aapt2_tests
Change-Id: I7b607192ecb337307c4bcb5770e7f716c6d4c9d2
diff --git a/tools/aapt2/process/SymbolTable.h b/tools/aapt2/process/SymbolTable.h
index 4a2181e..b676efb 100644
--- a/tools/aapt2/process/SymbolTable.h
+++ b/tools/aapt2/process/SymbolTable.h
@@ -199,6 +199,10 @@
   std::unique_ptr<SymbolTable::Symbol> FindByReference(
       const Reference& ref) override;
 
+  android::AssetManager* GetAssetManager() {
+    return &assets_;
+  }
+
  private:
   android::AssetManager assets_;