Clean up JNI calling convention callee saves.

Precalculate callee saves at compile time and return them
as ArrayRef<> instead of keeping then in a std::vector<>.

Change-Id: I4fd7d2bbf6138dc31b0fe8554eac35b0777ec9ef
diff --git a/compiler/jni/quick/jni_compiler.cc b/compiler/jni/quick/jni_compiler.cc
index 27714b8..4311a34 100644
--- a/compiler/jni/quick/jni_compiler.cc
+++ b/compiler/jni/quick/jni_compiler.cc
@@ -112,7 +112,7 @@
 
   // 1. Build the frame saving all callee saves
   const size_t frame_size(main_jni_conv->FrameSize());
-  const std::vector<ManagedRegister>& callee_save_regs = main_jni_conv->CalleeSaveRegisters();
+  ArrayRef<const ManagedRegister> callee_save_regs = main_jni_conv->CalleeSaveRegisters();
   __ BuildFrame(frame_size, mr_conv->MethodRegister(), callee_save_regs, mr_conv->EntrySpills());
   DCHECK_EQ(jni_asm->cfi().GetCurrentCFAOffset(), static_cast<int>(frame_size));