Supporting InsetDrawables and StateListDrawables with different states

[MrStahlfelge]

Our buttons use InsetDrawables for having an outer padding, this was not supported by the lib - it needed GradientDrawables. An InsetDrawable is a container that can hold an inner GradientDrawable.

This PR extends the lib to use the InsetDrawable as the background image while animating the inner GradientDrawable. I have unified the code for the two Progress Buttons.

You can test by using the following drawable xml as the button background:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">

	<!-- Normal Buttons State -->
	<item>
		<inset android:insetLeft="4dp"
			   android:insetTop="4dp"
			   android:insetRight="4dp"
			   android:insetBottom="4dp">
			<shape android:shape="rectangle">
				<corners android:radius="2dp"/>
				<solid android:color="@color/cpb_red"/>
				<padding
					android:left="8dp"
					android:top="4dp"
					android:right="8dp"
					android:bottom="4dp"/>
			</shape>
		</inset>
	</item>
</selector>

This branch is based on my other PR.