AppPickerPreference: changed format of how preference values are stored
Instead of constructing our own value from package name and class name,
take advantage of Intent.toUri() which converts intents into URI strings
which can be easily stored with persistString().
1) Simpler handling - we don't have to parse value ourselves
(Intent.parseUri() takes care of that)
2) Preparation for shortcut support - we now can store any intents
including special 
ones that can be used as shortcuts
3) Old preference values will be converted to new format automatically
so users don't need to set AppPicker preferences from scratch