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/process/SymbolTable.h b/tools/aapt2/process/SymbolTable.h
index e684bb0..43f4dd7 100644
--- a/tools/aapt2/process/SymbolTable.h
+++ b/tools/aapt2/process/SymbolTable.h
@@ -34,7 +34,7 @@
 namespace aapt {
 
 inline android::hash_t hash_type(const ResourceName& name) {
-    std::hash<std::u16string> strHash;
+    std::hash<std::string> strHash;
     android::hash_t hash = 0;
     hash = android::JenkinsHashMix(hash, (uint32_t) strHash(name.package));
     hash = android::JenkinsHashMix(hash, (uint32_t) name.type);