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/SdkConstants.h b/tools/aapt2/SdkConstants.h
index a6dba8b..f28679f 100644
--- a/tools/aapt2/SdkConstants.h
+++ b/tools/aapt2/SdkConstants.h
@@ -49,7 +49,7 @@
 
 size_t findAttributeSdkLevel(ResourceId id);
 size_t findAttributeSdkLevel(const ResourceName& name);
-std::pair<StringPiece16, int> getDevelopmentSdkCodeNameAndVersion();
+std::pair<StringPiece, int> getDevelopmentSdkCodeNameAndVersion();
 
 } // namespace aapt