Update learn more string of FooterPrefernce

Replace FooterPreference#SetLearnMoreContentDescription() with
FooterPreference#SetLearnMoreText().

This is a requirement: All links, controls, and buttons should
also have comprehensible text or tooltip text, so users can identify its
purpose, independent of context, such as when using the URL list from
TalkBack’s local context menu.

Bug: 215045903
Test: manual & robotest
Change-Id: Ib657ba336c5688c1434a58611dea3891001afe14