Force phan-taint-check to think LogFormatter stuff is safe for html

Force phan-taint-check to think LogFormatter stuff is safe for html

Whether or not LogFormatter::getActionText() and friends are safe
for html depends on the runtime value of LogFormatter::$plaintext
which is beyond the abilities of phan-taint-check's static analysis
to determine. Thus this method results in a lot of false positives.

To prevent that, add an annotation that marks the method as always
safe for html. This is not ideal, but refactoring this method while
maintaining back-compat looks like it would be very challenging.

Bug: T197002
Change-Id: I9aded350ed4acc733b4fb697dd3400686a178fa9