RequestManageCredentials Material Next Spec

* Update buttons to have rounded corners
  and more padding
* Update text colour of buttons
* Update floating action button background
  and text colour

Manual Testing via TestDPC
* Light mode https://screenshot.googleplex.com/8MxZZjftRFMXJhM
* Dark mode https://screenshot.googleplex.com/3vgWrEJnCbzUdhh
* Floating action button https://screenshot.googleplex.com/5H4nfTWz7sNJJmT
* Landscape

Bug: 189193577
Test: Manual testing
       RequestManageCredentialsTest
Change-Id: Iba8b4b6af655abfdab6216e6042ee222c3f745dd