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/process/SymbolTable.h b/tools/aapt2/process/SymbolTable.h
index 25f7565..cf597bb 100644
--- a/tools/aapt2/process/SymbolTable.h
+++ b/tools/aapt2/process/SymbolTable.h
@@ -154,7 +154,7 @@
  public:
   AssetManagerSymbolSource() = default;
 
-  bool AddAssetPath(const StringPiece& path);
+  bool AddAssetPath(const android::StringPiece& path);
 
   std::unique_ptr<SymbolTable::Symbol> FindByName(
       const ResourceName& name) override;