Save/restore local table state on down calls.

Also add unit test.

Change-Id: Ia1dc54eaac20c4bbb1ca3d9ac2933d6ab0241261
diff --git a/src/calling_convention_arm.h b/src/calling_convention_arm.h
index 3d513fe..984250e 100644
--- a/src/calling_convention_arm.h
+++ b/src/calling_convention_arm.h
@@ -42,6 +42,7 @@
   virtual const std::vector<ManagedRegister>& CalleeSaveRegisters() const {
     return callee_save_regs_;
   }
+  virtual ManagedRegister ReturnScratchRegister() const;
   virtual uint32_t CoreSpillMask() const;
   virtual uint32_t FpSpillMask() const {
     return 0;  // Floats aren't spilled in JNI down call