Remove using mirror::* in jni internal.

Cleans up the code since it helps make it easier to spot moving GC
related bugs.

Also fixed moving GC bugs in MonitorEnter, and FindFieldID.

Change-Id: I794e4fbcc2010875fc6b299c6d4c9fb0f071b71a
diff --git a/runtime/monitor.h b/runtime/monitor.h
index ca95e0b..d0a3a2e 100644
--- a/runtime/monitor.h
+++ b/runtime/monitor.h
@@ -57,7 +57,7 @@
   static uint32_t GetLockOwnerThreadId(mirror::Object* obj)
       NO_THREAD_SAFETY_ANALYSIS;  // TODO: Reading lock owner without holding lock is racy.
 
-  static void MonitorEnter(Thread* thread, mirror::Object* obj)
+  static mirror::Object* MonitorEnter(Thread* thread, mirror::Object* obj)
       EXCLUSIVE_LOCK_FUNCTION(monitor_lock_)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
   static bool MonitorExit(Thread* thread, mirror::Object* obj)