Fix issue where "learn more" link on 5G NR settings doesn't work.

Issue is due to the fact the footer text is a spannable string which
although different from the original text loaded when the preference
was constructed, is the same in terms of object equality.
Clearing the preference title before assigning the footer text fixes this.

Fixes: 162011372
Test: Manual test; open settings and verify link is clickable now.
Merged-In: I5d22c665b0d6fb842bc54dc55a2eb3045e0092a2
Change-Id: I5d22c665b0d6fb842bc54dc55a2eb3045e0092a2
(cherry picked from commit 97d8711d)
Exempt-From-Owner-Approval: Straight cherry-pick.