Add ripple effect to all buttons

[sarahsnow1]

Closes #459 #327

[ecgreb]

If we use <ripple> instead of <layer-list> this appears to produce the desired effect for all the buttons and as an added bonus the tests no longer crash. For example:

<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
    android:color="?android:colorControlHighlight">
    <item>
        <shape xmlns:android="http://schemas.android.com/apk/res/android"
            android:shape="rectangle">

            <solid
                android:color="@color/color_primary" />

            <stroke
                android:width="1dp"
                android:color="@color/gray" />
        </shape>
    </item>
</ripple>

[sarahsnow1]

You're right- looks like it does fix the tests! I had erroneously stayed away from ripple because i saw it introduced a requirement of minSdkVersion 21 however placing these in the qualified drawable folder doesn't require we bump the minSdkVersion.