(1) Followup to koreader/koreader-base#1921, #12503: when getting a pdf book id in the stats db, check for old records with null terminated authors and title.
Discussed in #11318. There are some other reasons for disappearing old stats for pdf (md5 changes when converting/writing to pdf or something else), hence not closing the issue.
(2) Add Dispatcher action for toggling statistics. Closes#11422.