Allow customization on the learn more text in FooterPreference

Before this change, the learn more text is hard-coded in the layout xml.
This change allows the caller to use an alternative text for the
learn more button.

Test: make RunSettingsLibRoboTests -j56 ROBOTEST_FILTER=com.android.settings.display.darkmode.DarkModeCustomBedtimePreferenceControllerTest
Bug: 215182463
Change-Id: Ic87afaeca595dc009c2070db666cd52b0ac1c9c8