Set CDC detail string as subtitle, not description

With an associated change to the UI of the BiometricPrompt credential
view, this commit preserves the current appearance of the CDC auth flow
by promoting the "details" string from the description to the subtitle
field of the prompt.

Test: Manually, using the TestDPC app

Bug: 152053691
Change-Id: If1d773f7f9a7b141520eac70a6cd64c09eb27f20