Add DexFileLoader class

Added DexFileLoader class, moved functionality from DexFile there:
- Multidex loading logic
- DexFile opening logic for Zip and etc
- Some other helpers

Bug: 63756964
Test: test-art-host

Change-Id: Ic3dfa458947d4b69912dea5cdd836e7e8f55061c
diff --git a/compiler/utils/test_dex_file_builder.h b/compiler/utils/test_dex_file_builder.h
index 8ef9882..e6501e0 100644
--- a/compiler/utils/test_dex_file_builder.h
+++ b/compiler/utils/test_dex_file_builder.h
@@ -26,6 +26,7 @@
 
 #include "base/bit_utils.h"
 #include "base/logging.h"
+#include "dex_file_loader.h"
 #include "native_dex_file.h"
 
 namespace art {
@@ -231,7 +232,7 @@
     static constexpr bool kVerify = false;
     static constexpr bool kVerifyChecksum = false;
     std::string error_msg;
-    std::unique_ptr<const DexFile> dex_file(DexFile::Open(
+    std::unique_ptr<const DexFile> dex_file(DexFileLoader::Open(
         &dex_file_data_[0],
         dex_file_data_.size(),
         dex_location,