Add ART_DEFAULT_COMPACT_DEX_LEVEL

If specified, this option changes the default compact dex level used
by dex2oat. Defaults to none currently.

Bug: 63756964
Test: ART_DEFAULT_COMPACT_DEX_LEVEL=fast mm test-art-host-gtest and verify it fails for now.
Test: test/testrunner/testrunner.py --host -j64 and verify it passes

Change-Id: Ib252f432b3545297725656a5cca40d1bb57b0ced
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 27bec1d..82331ba 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -2814,7 +2814,7 @@
   // Dex files we are compiling, does not include the class path dex files.
   std::vector<const DexFile*> dex_files_;
   std::string no_inline_from_string_;
-  CompactDexLevel compact_dex_level_ = CompactDexLevel::kCompactDexLevelNone;
+  CompactDexLevel compact_dex_level_ = kDefaultCompactDexLevel;
 
   std::vector<std::unique_ptr<linker::ElfWriter>> elf_writers_;
   std::vector<std::unique_ptr<linker::OatWriter>> oat_writers_;