Instead of showing the reminder after x unlocks, I decided to show the reminder
2 weeks after the vault was last unlocked with the password. Let me know if you
agree with that.

This adds an option to highlight tapped entries, so that it's easier to
distinguisch between the one you're trying to enter into a website and the other
ones.
Only one entry can be highlighted at a time. Perhaps it would make sense to
change our tap to reveal functionality to behave the same, so that the two
features are nicely in sync. I can address that in a separate PR if we decide to
do so.
This fixes the following bugs:
- Sort category is forgotten after lock/unlock
- The sort mode is not respected for new entries
I got a little carried away while working on this patch and also included the
following other enhancements:
- Simplify the SortCategory, Theme and ViewMode enums
- Simplify usage of string resources
- Don't call notifyDataSetChanged and runLayoutAnimation unnecessarily
This adds an option to automatically lock the app when:
* The back button is pressed
* The device is locked
It's the first step towards implementing #7