Add option to list all the keys saved in the DB

[dcgavril]

Listing all the keys saved could give you a possibility to clean the DB, without clearing everything (see .clear()). For example, I need to keep some login info or app settings, but just remove the rest after an app update or a requested clear cache/data, from the app (not from the apps manager - that clears everything even the settings). Or add an option to clear all, except a list of keys(that would be even better - less work for the app developer :) ).

Anyway, thank you for your work!

[orhanobut]

This seems a valid reason actually. Let me check PR. 👍

[KishorJena]

useful feature. I wanted to all keys in debugging. It could have save lots of time.