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/jni_cfi_test.cc b/compiler/jni/jni_cfi_test.cc
index 371019a..3526802 100644
--- a/compiler/jni/jni_cfi_test.cc
+++ b/compiler/jni/jni_cfi_test.cc
@@ -52,7 +52,7 @@
     std::unique_ptr<ManagedRuntimeCallingConvention> mr_conv(
         ManagedRuntimeCallingConvention::Create(&arena, is_static, is_synchronized, shorty, isa));
     const int frame_size(jni_conv->FrameSize());
-    const std::vector<ManagedRegister>& callee_save_regs = jni_conv->CalleeSaveRegisters();
+    ArrayRef<const ManagedRegister> callee_save_regs = jni_conv->CalleeSaveRegisters();
 
     // Assemble the method.
     std::unique_ptr<Assembler> jni_asm(Assembler::Create(&arena, isa));