Rewrite image type in CompilerOptions.

Prepare for introduction of boot image extension.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Change-Id: Id007c4aeb4ada84fe65c1148fdf67e8a412e5b74
diff --git a/compiler/driver/compiler_options.cc b/compiler/driver/compiler_options.cc
index 685cde3..3610f18 100644
--- a/compiler/driver/compiler_options.cc
+++ b/compiler/driver/compiler_options.cc
@@ -44,8 +44,7 @@
       no_inline_from_(),
       dex_files_for_oat_file_(),
       image_classes_(),
-      boot_image_(false),
-      app_image_(false),
+      image_type_(ImageType::kNone),
       compiling_with_core_image_(false),
       baseline_(false),
       debuggable_(false),