Fix handling of sqlite transactions (#136)

* Move as much work as possible out of the sqlite txn

In particular, don't do anything that could throw in the finally block,
as this will mask any exception thrown within the transaction itself.

* Use sqlite transaction listeners (rather than doing so manually)

Using sqlite's version allows us to not worry about getting the
transaction semantics correct in the face of exceptions.

Author: rsgowman

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLitePersistence.java

@@ -26,6 +26,7 @@
 import android.database.sqlite.SQLiteOpenHelper;
 import android.database.sqlite.SQLiteProgram;
 import android.database.sqlite.SQLiteStatement;
+import android.database.sqlite.SQLiteTransactionListener;
 import android.support.annotation.VisibleForTesting;
 import com.google.common.base.Function;
 import com.google.firebase.firestore.auth.User;
@@ -75,6 +76,21 @@ public static String databaseName(String persistenceKey, DatabaseId databaseId)
   private final SQLiteQueryCache queryCache;
   private final SQLiteRemoteDocumentCache remoteDocumentCache;
   private final SQLiteLruReferenceDelegate referenceDelegate;
+  private final SQLiteTransactionListener transactionListener =
+      new SQLiteTransactionListener() {
+        @Override
+        public void onBegin() {
+          referenceDelegate.onTransactionStarted();
+        }
+
+        @Override
+        public void onCommit() {
+          referenceDelegate.onTransactionCommitted();
+        }
+
+        @Override
+        public void onRollback() {}
+      };
 
   public SQLitePersistence(
       Context context, String persistenceKey, DatabaseId databaseId, LocalSerializer serializer) {
@@ -143,35 +159,32 @@ RemoteDocumentCache getRemoteDocumentCache() {
 
   @Override
   void runTransaction(String action, Runnable operation) {
+    Logger.debug(TAG, "Starting transaction: %s", action);
+    db.beginTransactionWithListener(transactionListener);
     try {
-      Logger.debug(TAG, "Starting transaction: %s", action);
-      referenceDelegate.onTransactionStarted();
-      db.beginTransaction();
       operation.run();
 
       // Note that an exception in operation.run() will prevent this code from running.
       db.setTransactionSuccessful();
     } finally {
       db.endTransaction();
-      referenceDelegate.onTransactionCommitted();
     }
   }
 
   @Override
   <T> T runTransaction(String action, Supplier<T> operation) {
+    Logger.debug(TAG, "Starting transaction: %s", action);
+    T value = null;
+    db.beginTransactionWithListener(transactionListener);
     try {
-      Logger.debug(TAG, "Starting transaction: %s", action);
-      referenceDelegate.onTransactionStarted();
-      db.beginTransaction();
-      T value = operation.get();
+      value = operation.get();
 
       // Note that an exception in operation.run() will prevent this code from running.
       db.setTransactionSuccessful();
-      return value;
     } finally {
       db.endTransaction();
-      referenceDelegate.onTransactionCommitted();
     }
+    return value;
   }
 
   /**