Unable to instantiate activity: java.lang.ClassNotFoundException: MainActivity in loader dalvik.system.PathClassLoader

My application was running fine on Android 4.x. When I tried testing in on an Android 2.3 emulator, I got the following error in logcat:

09-11 17:21:27.720    1546-1546/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.joelchristophel.romannumeralcalculator/com.joelchristophel.android.MainActivity}: java.lang.ClassNotFoundException: com.joelchristophel.android.MainActivity in loader dalvik.system.PathClassLoader[/data/app/com.joelchristophel.romannumeralcalculator-2.apk]
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1569)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
            at android.app.ActivityThread.access$1500(ActivityThread.java:117)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:130)
            at android.app.ActivityThread.main(ActivityThread.java:3683)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.ClassNotFoundException: com.joelchristophel.android.MainActivity in loader dalvik.system.PathClassLoader[/data/app/com.joelchristophel.romannumeralcalculator-2.apk]
            at dalvik.system.PathClassLoader.findClass(PathClassLoader.java:240)
            at java.lang.ClassLoader.loadClass(ClassLoader.java:551)
            at java.lang.ClassLoader.loadClass(ClassLoader.java:511)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1561)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
            at android.app.ActivityThread.access$1500(ActivityThread.java:117)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:130)
            at android.app.ActivityThread.main(ActivityThread.java:3683)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)

After looking around for help, I tried this suggestion. I am using the droidparts library, but had not included in AndroidManifest.xml. So I added it:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://ift.tt/nIICcg"
    package="com.joelchristophel.romannumeralcalculator"
    android:versionCode="7"
    android:versionName="1.1.1" >

    <uses-sdk
        android:minSdkVersion="9"
        android:targetSdkVersion="20" />

    <supports-screens
    android:anyDensity="true"
    android:largeScreens="true"
    android:normalScreens="true"
    android:resizeable="true"
    android:smallScreens="true"
    android:xlargeScreens="true" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">
       <uses-library
            android:name="org.droidparts"
            android:required="true" />
        <activity
            android:name="com.joelchristophel.android.MainActivity"
            android:label="@string/app_name"
            android:windowSoftInputMode="stateHidden"
            android:launchMode="standard"
            android:enabled="true" >

            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>