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/cmdline/cmdline_parser_test.cc b/cmdline/cmdline_parser_test.cc
index 5d67206..1e79fdf 100644
--- a/cmdline/cmdline_parser_test.cc
+++ b/cmdline/cmdline_parser_test.cc
@@ -375,7 +375,7 @@
 
 TEST_F(CmdlineParserTest, TestJdwpProviderDefault) {
   const char* opt_args = "-XjdwpProvider:default";
-  EXPECT_SINGLE_PARSE_VALUE(JdwpProvider::kInternal, opt_args, M::JdwpProvider);
+  EXPECT_SINGLE_PARSE_VALUE(JdwpProvider::kDefaultJdwpProvider, opt_args, M::JdwpProvider);
 }  // TEST_F
 
 TEST_F(CmdlineParserTest, TestJdwpProviderInternal) {