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

tools/aapt2/java/JavaClassGenerator_test.cpp

@@ -22,6 +22,8 @@
 #include "test/Test.h"
 #include "util/Util.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 TEST(JavaClassGeneratorTest, FailWhenEntryIsJavaKeyword) {