Progress Dialog causing LoginActivity to be leaked

[AmandaRiu]

The Issue
The LoginActivity is being leaked if the progress dialog is visible during a configuration change. Here is the stacktrace that is thrown:

WindowManager  E  android.view.WindowLeaked: Activity com.woocommerce.android.ui.login.LoginActivity has leaked window DecorView@a0f7aaf[] that was originally added here
             E      at android.view.ViewRootImpl.<init>(ViewRootImpl.java:485)
             E      at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:346)
             E      at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:93)
             E      at android.app.Dialog.show(Dialog.java:330)
             E      at android.app.ProgressDialog.show(ProgressDialog.java:185)
             E      at org.wordpress.android.login.LoginBaseFormFragment.startProgress(LoginBaseFormFragment.java:219)
             E      at org.wordpress.android.login.LoginEmailPasswordFragment.next(LoginEmailPasswordFragment.java:213)
             E      at org.wordpress.android.login.LoginEmailPasswordFragment$2.onClick(LoginEmailPasswordFragment.java:164)
             E      at android.view.View.performClick(View.java:6294)
             E      at android.view.View$PerformClick.run(View.java:24770)
             E      at android.os.Handler.handleCallback(Handler.java:790)
             E      at android.os.Handler.dispatchMessage(Handler.java:99)
             E      at android.os.Looper.loop(Looper.java:164)
             E      at android.app.ActivityThread.main(ActivityThread.java:6494)
             E      at java.lang.reflect.Method.invoke(Native Method)
             E      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
             E      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Steps to Reproduce

1. Install a fresh version of the app (or logout)
2. Login with email, then enter password and hit next - the progress dialog will show
3. Immediately change the orientation of the device
4. Observe error in the logs

Recommended Fix
Dismiss the dialog and drop the reference, for example in LoginBaseFormFragment:

    @Override public void onDestroyView() {
        if (mProgressDialog != null) {
            mProgressDialog.dismiss();
            mProgressDialog = null;
        }
        super.onDestroyView();
    }

Note: I recommend using mProgressDialog.dismiss() instead of mProgressDialog.cancel() to avoid doing unnecessary work defined in the OnCancelListener.