Distribute the padding between search&rec container and tabs/applist

When there are no recommendations (pre-V as well), the margins/paddings
between search container & apps list / tabs was 8dp extra.

Additionally, cleaning up -
1. requestLayout call on page switch is now not needed after limiting
the table space for recommendations in two pane picker.
2. After preview container sizes, TOP alignment actually looks a bit
better - and widget preview itself is automatically center aligned as
we club only same preview container sizes in a row now.

* http://screen/6e6YzjqXJrdZ7Bt
* http://screen/AVLpzoYSGpyr2rs
* http://screen/7xnmrtLvBMPX5iX
* http://screencast/cast/NjYyNzM4NDY3MjM4NzA3Mnw2NDY0MmU4ZC0xMw (phone)
* http://screencast/cast/NTYwMzE1NDI1ODI5NjgzMnw2NmM1MjRkZS0zNA (fold)
* http://screencast/cast/NTIzODk3NTM5MTMzNDQwMHxlYmE2NDEwNS0xZg (tablet)

Bug: 319152349
Flag: N/A
Test: Screenshot and manual
Change-Id: I164419624d02fb47e80ea844eea0ee2a869ad4df