Change well known thread methods to `ArtMethod*`.

Also rename `java_lang_ThreadGroup_removeThread` to
`java_lang_ThreadGroup_threadTerminated` in line with
the actual method it references.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Iaf2644d006f44759a47587c82db2dcdba0bfa67e
diff --git a/runtime/class_linker.cc b/runtime/class_linker.cc
index f76fe2a..919383f 100644
--- a/runtime/class_linker.cc
+++ b/runtime/class_linker.cc
@@ -1141,6 +1141,9 @@
       WellKnownClasses::java_lang_Integer_valueOf,
       WellKnownClasses::java_lang_Long_valueOf,
       WellKnownClasses::java_lang_Short_valueOf,
+      // Ensure `Thread` and `ThreadGroup` classes are initialized (avoid check at runtime).
+      WellKnownClasses::java_lang_Thread_init,
+      WellKnownClasses::java_lang_ThreadGroup_add,
       // Ensure `DirectByteBuffer` class is initialized (avoid check at runtime).
       WellKnownClasses::java_nio_DirectByteBuffer_init,
       // Ensure reflection annotation classes are initialized (avoid check at runtime).