Fix memory operations for history

Bug: 33106224
Test: manual - M+ and M- work on history entries
Change-Id: I38fb002265c4c95982f0c02af701cf547573cb83
diff --git a/src/com/android/calculator2/CalculatorResult.java b/src/com/android/calculator2/CalculatorResult.java
index 0b399c4..a677b5e 100644
--- a/src/com/android/calculator2/CalculatorResult.java
+++ b/src/com/android/calculator2/CalculatorResult.java
@@ -413,14 +413,14 @@
      * Add the result to the value currently in memory.
      */
     public void onMemoryAdd() {
-        mEvaluator.addToMemory(Evaluator.MAIN_INDEX);
+        mEvaluator.addToMemory(mIndex);
     }
 
     /**
      * Subtract the result from the value currently in memory.
      */
     public void onMemorySubtract() {
-        mEvaluator.subtractFromMemory(Evaluator.MAIN_INDEX);
+        mEvaluator.subtractFromMemory(mIndex);
     }
 
     /**