Use derived padding instead of static 300dp for large portrait displays

In tablet, portrait mode, a padding of 300dp was applied to make it
less elongated, but that doesn't scale well across different devices.

So, we use 1/6th of height as the additional padding. This fixes the
existing logic.

Bug: 318410881
Bug: 315055849
Flag: N/A
Test: Screenshot update in cl chain.
Change-Id: Ia9cfe481131f086b66f625069cbf9a7c0343c2c9