Integrate database with history.

Bug: 31623549

Note: This CL operates under the assumption that there is something
in the current expression field (handling for various states of
CalculatorDisplay in ag/1613446)

Change-Id: I70992067ddc9c5eec079f00604549727787e26fe
diff --git a/src/com/android/calculator2/CalculatorExpr.java b/src/com/android/calculator2/CalculatorExpr.java
index f20d2bf..c6eb9af 100644
--- a/src/com/android/calculator2/CalculatorExpr.java
+++ b/src/com/android/calculator2/CalculatorExpr.java
@@ -21,15 +21,12 @@
 import android.text.SpannableStringBuilder;
 import android.text.Spanned;
 import android.text.style.TtsSpan;
-import android.text.style.TtsSpan.TextBuilder;
 
-import java.math.BigInteger;
 import java.io.DataInput;
 import java.io.DataOutput;
 import java.io.IOException;
+import java.math.BigInteger;
 import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.IdentityHashMap;
 
 /**
  * A mathematical expression represented as a sequence of "tokens".