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.
Change-Id: I5d22c665b0d6fb842bc54dc55a2eb3045e0092a2