Use encrypted shared preferences from androidx.security

[tombailey]

This PR uses the EncryptedSharedPreferences class from the AndroidX Security Crypto library.

Pros:

• Remove the existing ad-hoc hybrid encryption approach in favour of Google's EncryptedSharedPreferences from the AndroidX Security Crypto library

Cons:

• The Release Candidate version of AndroidX Security Crypto library requires min sdk 23 (Android 6.0). Up until now, this library has been min sdk 16 (Android 4.1).

It seems the Alpha Release versions of AndroidX Security are min sdk 21 (Android 5.0)

I am not sure how you want to handle the min sdk change as it might require some apps that target older devices and use this library to stick with an older version. Perhaps it is worth waiting for Google to promote its 1.1.0-alpha to a release candidate. That should allow us to use min sdk 21 (Android 5.0) but still might be a problem for some apps.

[tombailey]

Hopefully, this will also address lots of the issues various people have seen with the keystore on different devices/Android versions

[tombailey]

Also, it is worth mentioning that because Android will use EncryptedSharedPreferences after this is merged, when developers push an update to their existing app, all previous data will be lost/ignored