Fix NPE caused by I34451000

It's unfortunate but some of the fields we cleared in I34451000 are accessed
in BG threads or after the activity is destroyed.  We could add != null checks
everywhere, but it'll be a mess.  I also think it's safer to simply remove
the "= null" lines.

On the other hand, clearing AsyncTasks are relatively safer because they are
kept only so that we can cancel them afterwards, so I kept them.  But let me
know if you want to revert the original CL.

Bug 2570603

Change-Id: I04a10dd7382bfcceb686c3e9af92f8949caf619e

src/com/android/email/activity/AccountFolderList.java

@@ -214,11 +214,9 @@ public class AccountFolderList extends ListActivity implements OnItemClickListen
         Utility.cancelTask(mDeleteAccountTask, false); // Don't interrupt if it's running.
         mDeleteAccountTask = null;
 
-        mListAdapter.changeCursor(null);
-        mListAdapter = null;
-
-        mHandler = null;
-        mControllerCallback = null;
+        if (mListAdapter != null) { 
+            mListAdapter.changeCursor(null);
+        }
     }
 
     public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

src/com/android/email/activity/MailboxList.java

@@ -199,9 +199,6 @@ public class MailboxList extends ListActivity implements OnItemClickListener, On
         mMessageCountTask = null;
 
         mListAdapter.changeCursor(null);
-        mListAdapter = null;
-        mHandler = null;
-        mControllerCallback = null;
     }
 
     public void onClick(View v) {

src/com/android/email/activity/MessageCompose.java

@@ -396,18 +396,13 @@ public class MessageCompose extends Activity implements OnClickListener, OnFocus
 
         if (mAddressAdapterTo != null) {
             mAddressAdapterTo.changeCursor(null);
-            mAddressAdapterTo = null;
         }
         if (mAddressAdapterCc != null) {
             mAddressAdapterCc.changeCursor(null);
-            mAddressAdapterCc = null;
         }
         if (mAddressAdapterBcc != null) {
             mAddressAdapterBcc.changeCursor(null);
-            mAddressAdapterBcc = null;
         }
-        mHandler = null;
-        mListener = null;
     }
 
     /**

src/com/android/email/activity/MessageList.java

@@ -328,9 +328,6 @@ public class MessageList extends ListActivity implements OnItemClickListener, On
         mSetFooterTask = null;
 
         mListAdapter.changeCursor(null);
-        mListAdapter = null;
-        mHandler = null;
-        mControllerCallback = null;
     }
 
     @Override

src/com/android/email/activity/MessageView.java

@@ -502,9 +502,6 @@ public class MessageView extends Activity implements OnClickListener {
             mMessageContentView = null;
         }
         // the cursor was closed in onPause()
-
-        mHandler = null;
-        mControllerCallback = null;
     }
 
     private void onDelete() {