feat: implement EncryptedSharedPreferences

[tuanbipa]

Change SharedPreferences to EncryptedSharedPreferences (Only supported for devices running on Android 6 or higher
)

[aparajita]

I don't think this is sufficient. If I'm not mistaken, this would render all existing data to be unreadable.

[lbrucher]

Hello, first off thank you both for the plugin and the PR :)
I understand that if we would merge that branch existing data would indeed not be readable anymore, which is not desirable.
An idea would be to keep the current implementation the default and add a new API method to allow users to use the EncryptedSharedPreferences instead.
Obviously that new API would only be making sense for Android but that does not seem like a show stopper to me.
What do you guys think?
Thanks!

[jimcase]

I just created a new PR continuing the work of @tuanbipa.
PR: #12