Add subtitle to settings (settings)

Allows a credential provider to show a
subtitle/summary underneath the title in
the list of providers.

Test: ondevice & atest
Bug: 253157366
Change-Id: I481da16985027a49ee464623b6db166d941cab55
(cherry picked from commit on googleplex-android-review.googlesource.com host: 6474e013)
Merged-In: I481da16985027a49ee464623b6db166d941cab55