Add title and description for recommended assistants

All of config_recommendedAssistantsBy,
config_recommendedAssistantsDescription and config_recommendedAssistants
need to be overlaid for the recommendation to be enabled.

FooterPreference is used instead of TopIntroPreference even for
non-footer description in order to maintain compatibility with previous
platforms and reduce risk.

Bug: 388234667
Flag: com.android.permission.flags.default_apps_recommendation_enabled
Test: manual with ag/31585104
Relnote: N/A
(cherry picked from commit f1b88d1558cd8014478f9722e5c6b3af748536d8)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:dc300cafb4bf822efc1e9668582f6fcf156157b9)
Merged-In: I3afb4ca99738b163b27c31f1781d4cea97bb82bf
Change-Id: I3afb4ca99738b163b27c31f1781d4cea97bb82bf