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/link/VersionCollapser_test.cpp

@@ -18,6 +18,8 @@
 
 #include "test/Test.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 static std::unique_ptr<ResourceTable> BuildTableWithConfigs(