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/ResourceUtils_test.cpp b/tools/aapt2/ResourceUtils_test.cpp
index f9c500b..048c692 100644
--- a/tools/aapt2/ResourceUtils_test.cpp
+++ b/tools/aapt2/ResourceUtils_test.cpp
@@ -48,8 +48,7 @@
   EXPECT_EQ(ResourceNameRef("android", ResourceType::kColor, "foo"), actual);
   EXPECT_TRUE(actual_priv);
 
-  EXPECT_FALSE(
-      ResourceUtils::ParseResourceName(StringPiece(), &actual, &actual_priv));
+  EXPECT_FALSE(ResourceUtils::ParseResourceName(android::StringPiece(), &actual, &actual_priv));
 }
 
 TEST(ResourceUtilsTest, ParseReferenceWithNoPackage) {