AAPT2: Remove usage of u16string

For legacy reasons, we kept around the use of UTF-16 internally
in AAPT2. We don't need this and this CL removes all instances of
std::u16string and StringPiece16. The only places still needed
are when interacting with the ResTable APIs that only operate in
UTF16.

Change-Id: I492475b84bb9014fa13bf992cff447ee7a5fe588
diff --git a/tools/aapt2/dump/Dump.cpp b/tools/aapt2/dump/Dump.cpp
index 8f0dd3a..88c6f64 100644
--- a/tools/aapt2/dump/Dump.cpp
+++ b/tools/aapt2/dump/Dump.cpp
@@ -137,8 +137,8 @@
         return nullptr;
     }
 
-    const std::u16string& getCompilationPackage() override {
-        static std::u16string empty;
+    const std::string& getCompilationPackage() override {
+        static std::string empty;
         return empty;
     }