Ensure the learn more string meet Accessibility requirement.

In Google Accessibility Rating, 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.

It's not enough clear for users that only show "Learn more", in previous
design we provided setLearnMoreContentDescription() let TalkBack speak
more information about "Learn more".
But the requirement including the text of the accessibility label and the
visible link text match.

So we should only set learnMore string that can ensure accessibility
label and text match, also show a relevant context in TalkBack's context menu.

Bug: 215045903
Test: manual
Change-Id: Id734733bb4272327bcfcc04db38c88fb5cd3131c