Bug fix for wallet screen and lockscreen icon.

1. Make the header icon imageview a fixed size.
2. Fix an NPE for when the card doesn't have a label text.
3. The lockscreen should request 1 maxCards instead of 2 (align with the Wallet Tile)

Test: manual
Bug: 187860480
Change-Id: Ic9b015c40260a44382a97ae02aa30cb0ee15a29d