several minor bugs and things listed below

bug:1648729
   print warning if a requery is attemped on main thread
bug:2459293
   browser death because of NoSuchElementException. don't use iterator
   to march thru the ArrayList. use old style for() loop.
bug:1609474
   don't allow downgrades of databases
other stuff
   use LRUcache to maintain statement-cache in SQLiteDatabase
Change-Id: I3a42022162f4dfcc75f7d0cfcad8f20f1d193249