Show Floating action button to request manage credentials

Background
* This is part of the work to support
  a credential management app on
  unmanaged devices.
Changes
* Show FAB for detailed/long manage credentials
  authentication policy.
* Hide FAB for short manage credentials
  authentication policy.
* Unexpand FAB once the user start starts
  scrolling.
* Hide FAB once the user has scrolled to
  the bottom.

Manual Testing
* Verify FAB is shown for a detailed/long policy:
  https://screenshot.googleplex.com/BUb4LGz3GD6AozS
* Verify FAB is hidden for a short policy
* Verify FAB is hidden when user has scrolled to the
  bottom:
  https://screenshot.googleplex.com/6FQRqto3r3jzfXH
* Verify FAB is unexpanded (text hidden)
  when the users start scrolling:
  https://screenshot.googleplex.com/4FfAt5MsCfrAwQK

Bug: 165641221
Test: Manual Testing
      make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.security.RequestManageCredentialsTest
Change-Id: Ied2ef726ad4dcc3f92c20249f80294f0a3071a8a