Refactor jdwp_provider to make it easier to change the default

Change JdwpProvider to have a kDefaultJdwpProvider member that can be
updated to be whatever the current default jdwp provider is.

Test: build
Test: cmdline_parser_test
Change-Id: I89559cc63d371be60b594d850f1a16a557b42edc
diff --git a/runtime/jdwp_provider.h b/runtime/jdwp_provider.h
index b62e10b..5f4a5ad 100644
--- a/runtime/jdwp_provider.h
+++ b/runtime/jdwp_provider.h
@@ -28,6 +28,9 @@
   kNone,
   kInternal,
   kAdbConnection,
+
+  // The current default provider
+  kDefaultJdwpProvider = kInternal,
 };
 
 std::ostream& operator<<(std::ostream& os, const JdwpProvider& rhs);