AAPT2: Support building, linking, and merging static libraries

Android static libraries are like APKs but they contain much more debugging
and source information. We need to treat them differently in 3 ways:

1) When building a static library, we skip things like ID assignment and
   product/config stripping. Source information is emitted as well.
2) When building a static library and linking against another
   static library, we don't want to merge, we want to simply reference.
3) When building an app that uses static libraries, we want to merge
   the static library under the same package with or without mangling.

Bug:25958912
Change-Id: I425e032857936a3e83173c1edc2a6cdc6020b842
diff --git a/tools/aapt2/process/IResourceTableConsumer.h b/tools/aapt2/process/IResourceTableConsumer.h
index 3a88044..9affb83 100644
--- a/tools/aapt2/process/IResourceTableConsumer.h
+++ b/tools/aapt2/process/IResourceTableConsumer.h
@@ -30,14 +30,14 @@
 namespace aapt {
 
 class ResourceTable;
-struct ISymbolTable;
+class SymbolTable;
 
 struct IAaptContext {
     virtual ~IAaptContext() = default;
 
-    virtual ISymbolTable* getExternalSymbols() = 0;
+    virtual SymbolTable* getExternalSymbols() = 0;
     virtual IDiagnostics* getDiagnostics() = 0;
-    virtual StringPiece16 getCompilationPackage() = 0;
+    virtual const std::u16string& getCompilationPackage() = 0;
     virtual uint8_t getPackageId() = 0;
     virtual NameMangler* getNameMangler() = 0;
     virtual bool verbose() = 0;