Installation error: INSTALL_FAILED_OLDER_SDK

This means the version of android of your avd is older than the version being used to compile the code

It is due to android:targetSdkVersion="@string/app_name" in your manifiest file.
Change it to:

<uses-sdk android:minSdkVersion="15" android:targetSdkVersion="15"/>

The targetSdkVersion should be an integer, but @string/app_name would be a string. I think this causing the error.

EDIT:
You have to add a default intent-filter in your manifiest file for the activity. Then only android can launch the activity. otherwise you will get the below error in your console window.

[2012-02-02 09:17:39 - Test] No Launcher activity found!
[2012-02-02 09:17:39 - Test] The launch will only sync the application package on the device!

Add the following to your <activity> tag.

<activity android:name="HelloAndroid" 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>