Fix Page styles.

* Simplified billing.xml layout.
Use Page.Centered for migration activity layout.

* Removed some superfluous attributes.
Add w590dp variant for Page.Centered.

* Added height attribute in w400dp style for convenience.
Use Page.Centered in data liberation and stats fragment.
Remove unused CenteredWrappingColumn style.
Use Page.Centered in legacy backup and trakt sync layouts.

* Also cleaned up backup and trakt sync layouts a little.
Removed unused CenteredColumn style.