ART: Rewrite core image detection.

Remove unused compiler option `core_image_` and cache
the IsCompilingWithCoreImage in the CompilerOptions instead.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Idcf3b6c96dab9a39f823a16778e7698589442cd8
diff --git a/compiler/driver/compiler_options.cc b/compiler/driver/compiler_options.cc
index be8e10e..685cde3 100644
--- a/compiler/driver/compiler_options.cc
+++ b/compiler/driver/compiler_options.cc
@@ -45,8 +45,8 @@
       dex_files_for_oat_file_(),
       image_classes_(),
       boot_image_(false),
-      core_image_(false),
       app_image_(false),
+      compiling_with_core_image_(false),
       baseline_(false),
       debuggable_(false),
       generate_debug_info_(kDefaultGenerateDebugInfo),