Allow to stylise flexboxLayoutManager from XML

[vyguera]

It is possible to use layoutmanager attribute in layout xmls to use FlexboxLayoutManager as following:

<androidx.recyclerview.widget.RecyclerView xmlns:android="http://schemas.android.com/apk/res/android"
    app:layoutManager="com.google.android.flexbox.FlexboxLayoutManager">

The same way you can set that attribute in a style this way

<androidx.recyclerview.widget.RecyclerView xmlns:android="http://schemas.android.com/apk/res/android"
    android:style="@styles/MyFlexListStyle">

<resources>
    <style name="MyFlexListStyle">
        <item name="layoutManager">com.google.android.flexbox.FlexboxLayoutManager</item>
    </style>
</resources>

These PR allows you to develop a library that provides with a view/fragment/screen with a recyclerView that is styleable from the app the same way as the regular FlexboxLayout does, except for any already handled attributes (flexDirection) or unsupported ones (alignContent), so you can do the following:

<androidx.recyclerview.widget.RecyclerView xmlns:android="http://schemas.android.com/apk/res/android"
    android:style="@styles/MyFlexListStyle">

<resources>
    <style name="MyFlexListStyle">
        <item name="layoutManager">com.google.android.flexbox.FlexboxLayoutManager</item>
        <item name="flexWrap">wrap</item>
        <item name="justifyContent">center</item>
        <item name="alignItems">baseline</item>
        <item name="maxLine">3</item>
    </style>
</resources>

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