Example of pretty simple implementation of progress bars in Android based on ClipDrawable
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
<item>
<bitmap android:src="@drawable/pb_empty"/>
</item>
<item>
<clip
android:clipOrientation="vertical"
android:gravity="bottom"
android:drawable="@drawable/pb_full"/>
</item>
</layer-list>