Move StringPiece to libandroidfw

libandroidfw needs to make use of StringPiece, so
move it to libandroidfw and update all code referencing
StringPiece in aapt2.

Test: make libandroidfw_tests libaapt2_tests
Change-Id: I68d7f0fc7c651b048d9d1f5e7971f10ef5349fa1
diff --git a/tools/aapt2/test/Context.h b/tools/aapt2/test/Context.h
index 7986329..63e5f16 100644
--- a/tools/aapt2/test/Context.h
+++ b/tools/aapt2/test/Context.h
@@ -70,8 +70,8 @@
 
 class ContextBuilder {
  public:
-  ContextBuilder& SetCompilationPackage(const StringPiece& package) {
-    context_->compilation_package_ = package.ToString();
+  ContextBuilder& SetCompilationPackage(const android::StringPiece& package) {
+    context_->compilation_package_ = package.to_string();
     return *this;
   }
 
@@ -103,9 +103,8 @@
 
 class StaticSymbolSourceBuilder {
  public:
-  StaticSymbolSourceBuilder& AddPublicSymbol(
-      const StringPiece& name, ResourceId id,
-      std::unique_ptr<Attribute> attr = {}) {
+  StaticSymbolSourceBuilder& AddPublicSymbol(const android::StringPiece& name, ResourceId id,
+                                             std::unique_ptr<Attribute> attr = {}) {
     std::unique_ptr<SymbolTable::Symbol> symbol =
         util::make_unique<SymbolTable::Symbol>(id, std::move(attr), true);
     symbol_source_->name_map_[ParseNameOrDie(name)] = symbol.get();
@@ -114,7 +113,7 @@
     return *this;
   }
 
-  StaticSymbolSourceBuilder& AddSymbol(const StringPiece& name, ResourceId id,
+  StaticSymbolSourceBuilder& AddSymbol(const android::StringPiece& name, ResourceId id,
                                        std::unique_ptr<Attribute> attr = {}) {
     std::unique_ptr<SymbolTable::Symbol> symbol =
         util::make_unique<SymbolTable::Symbol>(id, std::move(attr), false);